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Excerpts From

The Granites of Maine, Bulletin 313

By T. Nelson Dale

(Continued)

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Dikes, Basic.

Dikes of dark-greenish or black, hard and dense rock (diabase, rarely basalt) are of very common occurrence in the Maine quarries.  The courses of some of these dikes and their relation to the joints are shown in figs. 9, 10, 14, 31, 34, 36, 38, and 39.  The courses of 23 dikes are distributed as follows:

Courses of 23 basic dikes at Maine granite quarries.
N. 10-12 E. 3
N. 22-30 E. 5
N. 40-50 E. 6
N. 70 E. 1
E.-W. 2
N. 15-30 W. 4
N. 45 W. 1
N. 75 W. 1

The northeast, north-northeast, and north-northwest courses are thus the most common.  The dikes are vertical, or nearly so, and range in width from 1 inch to 7 feet or more, cutting the granite sheets with mathematical definiteness.  Pl. VIII, B, shows one of these dikes on Mount Desert, which has a course N. 15 W., and has been faulted from east to west, or west to east, along a gently inclined sheet with a displacement of 16 inches.  A few feet beyond this point the same dike has been faulted along a northeast-southwest vertical joint with a displacement of 5 feet.  Although it might seem that this dike was injected into the granite before the sheet structure was formed, it is quite possible that the sheet structure preceded the dike and that at a later time faulting affected both the sheets and the dike, cracking the dike along the sheets when it did not actually fault it.

Most of these dikes are so firmly welded to the granite that hand specimens that are one-half granite and one-half diabase are readily obtained.  Thin sections of the glassy rims of dikes at Bryant Pond and Fryeburg show that the dike set out microscopic branches for short distances into the granite, in places surrounding some of its quartz particles.  A dike at the Dunbar Brothers' quarry, near Sullivan (p. 113), measuring from 16 to 18 inches in width, has a quarter-inch border of light-green epidote, derived from the alteration of its glassy rim.  These glassy borders are due to the rapid cooling of the material at its contact with the cold granite.  A vertical diabase like in Franklin (see p. 91) has darkened the shade of the granite and filled it with low-dipping close joints for a space of 10 feet on each side.  Under the microscope the quartz particles and some of the feldspars show parallel cracks 0.25 to 1.25 mm. apart.  A few typical thin sections of these dikes will be described in detail.

The center of a 7-foot dike at the Mosquito Mountain quarry near Frankfort (p. 153) shows a network of minute lath-shaped crystals of lime-soda feldspar (labradorite) partly altered to a white mica, in the meshes of which is a green hornblende; also some magnetite in fine particles and pyrite, with accessory titanite, apatite, and secondary epidote.

A 2 1/2-foot dike at Campbell & Macomber quarry, on the west side of Somes Sound, on Mount Desert (p. 99), shows a groundmass of fine hornblende and feldspar (plagioclase) in incomplete crystals, with magnetite (?), pyrite partly altered to limonite, and lime-soda feldspar (labradorite).  Some of the particles of the latter measure up to one-tenth inch in length by one-fiftieth inch in width and are largely altered to a kaolin and a white mica.  The hornblende in all of these dikes is regarded as a product of the alteration of augite.

The geological age of these dikes has not been precisely determined.  They are considerably more recent than the granite they traverse or the dikes of aplite and pegmatite which traverse the granite.  In Pl. XI, A, one of these diabase dikes is shown crossing both a vein of pegmatite and a mass of diorite ("black granite"), at Round Pond, in Lincoln County.[32]  (See p. 139.) 

The diabase dikes are the result of an earth movement that either opened previously formed joints or made new ones deep enough to be injected with volcanic material.  How far this may have penetrated the rocks which overlay the granite or whether it overflowed at their surface cannot even be conjectured.

At the granite quarries, wherever this course is possible, the dikes and the headings are left to form the bounding walls of the excavations.

Segregations (Knots).

Quarrymen know too well that granite is often disfigured by gray or black "knots" of circular or oval irregular curved outline, ranging in diameter from half an inch to 3 feet and exceptionally even 10 feet.  These were studied by geologists long ago.[33]  They are finer grained than the granite in which they occur, contain nearly 10 per cent less silica, much more black mica or hornblende (which accounts for their darkness), and a little more soda-lime feldspar, and their specific gravity is about 0.09 per cent higher.

Pl. V, A, shows 12 knots in the vertical cuts at Crabtree & Havey's quarry, in Sullivan.  As the strength and durability of the stone are in no wise affected by the "knots," the blocks containing them are used for curbing, crossings, or other constructions where color and shade are not taken into consideration.

A thin section of a very dark gray knot from this quarry shows a much greater abundance of biotite than the granite.  The feldspar and the biotite particles in the knot measure up to 0.5 mm. (one-fiftieth inch), whereas in the granite the feldspar measures up to 2.25 mm. and the biotite up to 0.75 mm.

A knot from the Palmer quarry, on Vinalhaven, is of medium-gray shade, with a very fine grained groundmass inclosing porphyritic buff-pinkish feldspars and smoky quartz particles up to about one-fourth inch in diameter.  The groundmass consists of quartz, potash feldspar (microcline), soda-lime feldspar, black mica, and hornblende in particles ranging in size from 0.075 to 0.5 mm.  The porphyritic particles are of quartz, potash feldspar, or soda-lime feldspar, and hornblende, and measure from 075 mm. up.

A very dark, almost black, knot from the Sands quarry, on Vinalhaven, consists of crowded particles of hornblende and biotite, which compose one-half the knot, the rest being mostly soda-lime feldspar and quartz.  Among the knots noticed is a spherical one, 2 1/2 feet in diameter, at the Sands quarry, and a similar one, 5 feet in diameter, at the Webster quarry, both in Vinalhaven.  One at the Mount Waldo quarry measures 6 by 3 feet and consists of a medium-gray groundmass with porphyritic feldspars up to three-fourths inch and biotite scales up to one-twentieth inch.  One at the Andrews & Perkins quarries, near Biddeford (p. 179), is 10 feet long.  At another Biddeford quarry the knots are egg-shaped and occur in clusters.  At the Tayntor quarry, in Hallowell, there is a belt 5 to 25 feet wide, with a course N. 10 E., in which knots are abundant.  This crosses the flow structure, which strikes N. 35 W.

In none of the knots is there a definite boundary separating them from the granite, excepting such as is caused by the change in the proportionate abundance of the darker minerals.  The cause of knots is not perfectly understood.  They are collections (segregations) of the darker, heavier, iron-magnesia minerals that took place while the rock was in a plastic state.

Geodes.

Small cavities lined with crystals occur in granite.  They are uncommon in the Maine quarries, but at the Bodwell granite Company's quarry, near Jonesboro (p. 169), there are several about a foot in diameter, lined with quartz crystals and epidote.  The center of some of these is filled with calcite (lime carbonate) in very obtuse rhombohedra half an inch across.  The large aplite vein at the same quarry has many irregular openings lined with crystals of feldspar and muscovite.  At the Machias Granite Company's quarry, near Marshfield (p. 174), there are several geodes, up to 6 inches in diameter, lined with crystals of feldspar and amethyst, with the central space filled with chlorite, epidote, and calcite.

Such cavities are attributed to bubbles of steam or gas that were in the rock while it was in a molten state, which gave room for the growth of crystals and later became filled with epidote and calcite.

Inclusions.

Not to be confounded with "knots," although some of them are equally dark and occur near them, are irregular or angular particles of various schistose rocks which the granite incorporated into itself during its intrusion.  They can usually be distinguished from the knots by their different microscopic structure.  Inclusions of this kind occur here and there in the Maine quarries.  Thus, at the Stimson quarry, in West Sullivan (p. 111), they measure about 1 inch, more or less, across, and consist of a fine-grained plicated biotite schist with a very little andesine feldspar.

But inclusions also occur on a large scale.  Thus, at the Freeport quarry (p. 78), 30 feet below the surface of the granite and completely surrounded by it, is a mass of biotite schist between 30 and 40 feet long and 3 feet thick, striking north and dipping 35 east.  In quarrying, this mass has been cut from east to west.  Under the microscope this is a coarse biotite quartz and feldspar (oligoclase) schist.  It is probably of sedimentary origin.  Pl. VII, B, shows part of the jagged edge of the lower portion of the inclusion and two isolate fragments of it in the granite.  The granite below one of the protruding angles of the inclusion is badly stained by ferruginous water coming from the schist.  In some parts of this inclusion there are streaks of pegmatite consisting of feldspar (oligoclase-andesine) and quartz.  The period of its formation is uncertain.

Contacts.

At the Waldoboro quarry the original contact of the upper part of the granite mass with the lower part of the remnant of the schist mass, which once overlay that region and into which the granite was intruded, is exposed.  (See Pl. IX, A, and p. 142.)  This schist is a hornblende-biotite-quartz schist containing some andesine feldspar, also accessory titanite and zircon. It is a metamorphosed rock, probably of sedimentary origin.  At the opposite or southwest end of the quarry (see fig. 29) the relations between the schist and granite are very complex, and a considerable mass of pegmatite intervenes in places.  The granite sends small dikes into the schist and also contains inclusions of it.  The granite was erupted after or during the folding of the schist, otherwise it would have become a gneiss.

Minerals on Joint Faces.

Joint faces in granite are in some places coated with minerals which do not occur in the granite itself or but very sparsely.  At the Sands quarry, in Vinalhaven, one of the joint faces bears very minute crystals of stilbite, a hydrous silicate of alumina, lime, and soda,[34] also hematite.  In other places the face is coated with a film of crystalline calcite from one-tenth to one-fifth inch thick.  Calcite occurs also similarly at one of the Redbeach quarries.  (See p. 166.)  A thin coating of secondary fibrous muscovite or of epidote occurs at several quarries.  At the W. B. Blaisdell quarry, in Franklin, certain joints are coated with crystalline calcite to a thickness of one-fourth inch, forming in places banded veins.  (See p. 41.)  A thin section of the granite away from the joint does not show any carbonate, but Mr. E. C. Sullivan, of the Survey (p. 94), found 0.24 per cent of lime carbonate in a few ounces of the same specimen.  Other joints in the same quarry with a different strike are coated with pyrite, and from their rusty appearance are known by the quarrymen as "iron seams."  At the Bryant Pond quarry (see p. 146) one of the joints is coated with calcite (with a little epidote and pyrite) up to half an inch in thickness, and the granite on either side contains considerable chlorite, derived probably from the alteration of its hornblende.  A muscovite-biotite granite quarried at Oxford (p. 146) has considerable secondary muscovite developed along planes which appear to be due to close jointing.  At the McMullen quarry, on Somes Sound, Mount Desert, the light buff and white feldspar is altered for the width of a foot along the steep joints to a deep reddish color.  This change does not occur along the sheets.  A thin section of this red feldspar shows that the color affects both potash and soda-lime feldspars alike and is due to innumerable dots of infinitesimal size, but without definite form or color under the higher powers of the microscope.  They are probably hematite.

It seems probably that the calcite and pyrite are infiltrations from calcareous and ferruginous formations that once overlay the granite, but were subsequently eroded.[35]

The presence of epidote, chlorite, muscovite, stilbite, pyrite, and hematite in or near joint faces may be attributed to a process of deep-seated mineral alteration aided by percolating waters, which too, up some elements and deposited others, and were also probably under pressure.  These changes may have occurred subsequent to the intrusion of the diabase dikes, because the dikes also have suffered similar alteration.

Discoloration ("Sap," Etc.).

Rusty (limonite) staining along the upper and lower parts of the sheets and also along the joints and headings is common in granite quarries, although some quarries are almost entirely free from it.  The concentric inward growth of "sap" from the close joints of a heading is well shown in Pl. IX, B.  The zone of discoloration along the sheets in the Maine quarries is from one-half to 12 inches, exceptionally even 18 inches, wide on each side of the sheet parting.  Its width, however, decreases gradually from the surface sheets downward.  In places the sap consists of two parts-an outer dark brownish zone from three-fourths to 1 1/2 inches wide and an inner more yellowish zone from one-fourth to one-half inch wide.  Generally, however, the discoloration diminishes gradually from without inward.  In some quarries there seems to be a connection between the "shake" structure (p. 150) and the discoloration, since these are coextensive.

When the stone is intended for facing or trimming buildings the presence of sap is a serious matter, as the stained edge of each block must be split off, which adds somewhat to the cost of production.

This discoloration has been supposed to be always due to the oxidation of the ferruginous minerals of the granite, biotite, hornblende, magnetite, and pyrite, but the Maine thin sections examined by the writer do not bear out this theory.  Thus one from the Tayntor quarry, near Hallowell, shows that the stain has insinuated itself into the cleavage planes and cracks of the feldspar and muscovite and in the cracks of the quartz, forming minute deposits of limonite therein, but the biotite scales and magnetite particles are generally untouched by the stain.  A section taken from the "top" of the Hopewell quarry, in Sullivan, where the fresh rock has a bluish tinge and the sap a general buff color, shows that the staining extends along the cleavages and fissures in the spaces between the minerals, but that it does not appear in connection with the biotite scales, although it is increased by the magnetite particles.  A section from the upper part of High Isle, south of Rockland, where the dark sap is an inch wide and an inner lighter part is one-fifth inch wide, shows a series of roughly parallel cracks crossing the sap vertically, with subsidiary transverse cracks.  These cracks and the cleavages of the feldspar and the spaces between the minerals are stained, but the staining has no connection with the biotite, and some large particles of magnetite are scarcely touched by it.  (See fig. 2.)  In the outer zone the limonite is darker and probably older and thicker than in the inner one.  That "sap" is not generally due to the oxidation of the minerals of the granite is also probable from the fact that no such general discoloration appears on fresh granite surfaces, even after many years of exposure to the weather. 

Fig. 2.  Minerals in thin section (3.76 by 4.23 millimeters = 0.15 by 0.17 inch) of biotite granite from High Isle in Knox County, showing "sap."  The ramifications of "sap" (limonite stain) across and around feldspar and quartz particles (marked f and g) are independent of the biotite and magnetite particles.  Fine-lined parts are biotite; fine-dotted areas are titanite; large black masses are magnetite.  Some of the borders of quartz particles are shown by dotted lines.

Fig. 2.

These observations lead to the inference that the discoloration called "sap" is, in the Maine granites, not due chiefly to the oxidation of the ferruginous minerals of the granite by "underground water," but chiefly to the deposition of limonite by ferruginous surface water.  The water descended along the vertical joints and then flowed along the sheet partings and permeated the rock above and below them.  This staining near the surface is intimately associated with the "shake" structure, which may be the result of frost.  Whether the postglacial submergence of the Maine coast had anything to do with the discoloration is not clear.

Another kind of discoloration, which is even more serious in its consequences, appears on fresh faces of granite, either in the quarry or after its removal.  This consists of sporadic rusty stains from half an inch to 1 inch in diameter, arising from the oxidation of minute particles of some undetermined ferruginous mineral.  In the Maine quarries these limonitic spots are very exceptional.

Daly[36] describes a bluish-gray syenite (feldspar, quartz, hornblende, augite, biotite) that after twenty-four hours' exposure assumes a greenish tinge, which eventually becomes more or less brownish.  He has demonstrated by experiment with oxygen that this change is due to the oxidation of minute blackish granules of ferrous oxide within the feldspar, giving a yellow which, in combination with the original bluish tint of the feldspar, produces a green.  The large columns of the library of Columbia University, in New York, are made of this rock.  Such changes, however, are uncommon in granitic rocks.  The only similar one observed in the Maine granites was in the quartz diorite of Alfred, in York County.  (See p. 175.)

Another kind of discoloration occurs on either side of diabase or basalt dikes, consisting mainly of various alterations of the feldspars, and their consequent change in shade or color.  (See p. 91.)

Discoloration is thus of four kinds:  That due to the infiltration of ferruginous water, that due to the oxidation of sporadic ferruginous minerals, that arising from the oxidation of ferrous oxide within the feldspars, and that due directly or indirectly to dikes and veins.  To these should be added a possible fifth-that due to the oxidation of the generally disseminated ferruginous minerals (biotite, hornblende, magnetite) by nonferruginous water.

Decomposition.

Notwithstanding the strength and durability of granite, it is liable, under certain conditions and in the course of long time, to decompose into clayey sand.  This is the result of its physical, mineralogical, and chemical constitution and properties.  One of the most striking illustrations of this is the occurrence in some of the Maine quarries of "beds" of sand or decomposed granite within the fresh granite, either between the sheets away from headings or within the headings and along or across the sheets.  Thus at the Palmer quarry, in Vinalhaven, 20 feet below the surface in the face of the quarry there is a bed of granite sand 18 inches thick between two sheets, which at that point dip about 10 into the hill.  On the southeast side of the Longfellow quarry, near Hallowell, some of the sheets within a wide heading include granite sand beds 10 inches thick.  At the Shattuck Mountain quarry, near Redbeach (see p. 165) a 6-foot heading includes a vertical layer of granite sand 8 inches thick.  Specimens taken from these various sand beds show that the disintegration begins with miscroscopic (sic) fractures; in some cases the enlarged rift cracks, producing the "shake" structure described on page 40,and is followed by more or less kaolinization of the feldspars.  This process consists in the loss of alkali and the taking up of water, resulting in the passing of the feldspar into a white clay (kaolin).

The joint and sheet structure affords ingress to surface water, containing its usual percentage of carbonic acid, and the "rift" or "shake" structure facilitates the kaolinization of the feldspar on either side of the sheet parting by this water.  As the feldspars pass into clay the rock crumbles into sand consisting of quartz, mica, and kaolin, and of feldspar in various stages of kaolinization.  In some places within the range and depth of frost a large part of this work may have been done by frost alone.  The sand would then be mainly the product of the "shake" structure.

In regions which have not been swept by a continental glacier any granite mass would be covered with the products of the decomposition of its own surface.  In the Tropics the abundant rainfall and the organic acids from a luxuriant vegetation materially hasten the decomposition, and the granitic rocks in such regions are for these reasons often covered with many feet of sand and soil.[37]  Along the Maine coast the surface of granite ledges bear in protected places an inch or so of granite sand, which represents surface disintegration since the postglacial submergence.

The incipient stage of weathering may be observed in any long-exposed granite ledge in the milky whiteness of the feldspars.  This change usually attacks the soda-lime feldspars first.  The black mica, owing to its content of iron oxide, is also liable to early decomposition.  The process of weathering, as it affects the rock as a whole, involves the following chemical changes:  A loss of lime, magnesia, potash, and soda; a gain of water, and a relative gain of silica, alumina, and iron oxide-that is, relative to the reduced weight of weathered rock. The subject of weathering of granite is fully treated in the writings of Merrill, Keyes, and Watson.[38]

The changes in granite after it has entered into buildings or other constructions are less marked than those in the natural rock, because the blocks are not then traversed by anything analogous to sheet and joint structure, and also because the years of historic time are few compared to those of geologic time.  Much has been written on the decay of granite in monuments and buildings.[39]  Such decay is mainly attributable to microscopic fissures produced by the unequal and repeated expansion and contraction of the different minerals of the granite under changes of solar temperature.  In countries where the winter temperature is very low the action of frost within such fissures powerfully assists the process of disintegration.  Thus the obelisk now in New York suffered more from three years' exposure to our climate than it had during over three thousand four hundred years in Egypt, although the fissures along which frost operated were started long before it reached this country.  A minor factor in decay is the chemical action of water along fissures.[40]  It is supposed that these causes of decay operate more effectively in coarse granites than in fine ones.  Merrill points out that a sawn or properly prepared polished surface resists weathering more effectively than a cut or hammered one, as the latter is full of minute fractures, parallel to the surface, produced by impact, which facilitate scaling.

Black Granites.

Black Granites in General.

Classification.

The term "black granites," although sufficient for general commercial purposes, includes a variety of rocks of different character, origin, and appearance-gabbros, diorites, diabase, etc.  They have, however, three mineralogical features in common-they contain comparatively little or no quartz, their feldspar belongs entirely or almost entirely to the series which contains both soda and lime, and they contain a considerable amount of one of the pyroxenes, or hornblende or biotite, and magnetite, which accounts for the general darkness of their shade or their greenish color.

Origin.

The gabbros and diorites are more or less granitic in texture, as they crystallized under conditions resembling those which attended the formation of granite.  But the diabase was in part erupted through narrow fissures, forming dikes or sheets, and at many places reached the surface, always crystallizing with comparative rapidity. 

Diabase, however, occurs in Vinalhaven, as stated by Dr. George Otis Smith, "in large bodies which have the form of neither dikes nor sheets, being, in fact, part of the same masses as the diorites and gabbros."

Mineralogical and Chemical Composition.

Gabbro consists essentially of a lime-soda feldspar and one or both of the varieties of pyroxene known as diallage and hypersthene.  The former is a foliated silicate of iron and lime with about 12 per cent of magnesia; the latter is a silicate of iron with about 24 per cent of magnesia, and each of these minerals crystallizes differently.  When hypersthene alone is present the rock is called a norite; when both are present it is a hypersthene gabbro.  When the mineral olivine (a greenish silicate of iron with 50 per cent of magnesia) is present also the name olivine may be prefixed to the rock name.  The accessory minerals in gabbros are ilmenite (a titanate of iron), magnetite, pyrite, apatite, biotite, garnet, and, rarely, quartz and metallic iron.  The secondary minerals-that is, those derived from the alteration of the primary ones-are hornblende, chlorite, epidote, zoisite, analcite, serpentine, a white mica, and calcite.  The percentage of silica in gabbros varies a little on either side of 50.  Iron oxides and lime average 9 per cent each; magnesia, 6 per cent.

Diorite consists essentially of feldspar (of the series containing lime and soda) and hornblende with biotite, or biotite alone.  Quartz, augite, and potash feldspar may or may not be present.  The accessory minerals are magnetite, pyrite, titanite, zircon, apatite, garnet, allanite.  The secondary are epidote, chlorite, a white mica, and calcite.  When quartz is present the rock is called a quartz diorite.  When black mica or augite are the preponderating iron-magnesium silicates the rock becomes a mica diorite or an augite diorite.  In diorites the silica ranges from about 49 to 63 per cent, but in quartz diorite it rises to about 69 per cent, which is the minimum in granite.  The iron oxides range from 0.52 to 9.70 per cent, the magnesia from less than 1 to over 11 per cent, but usually from 2 to 7 per cent.

Diabase consists essentially of a feldspar of the series containing lime, or soda and lime, together with a pyroxene or augite (alumina, lime, magnesia, iron), which, however, is frequently altered to hornblende or other secondary minerals; also magnetite or ilmenite or both.  Olivine may or may not be present, and some specimens contain a little quartz.  The accessory minerals are orthoclase, biotite, pyrite, hypersthene, apatite.  The secondary ones are hornblende, a white mica, chlorite, epidote, serpentine, calcite.  The percentage of silica in diabase ranges from about 45 to nearly 57, or iron oxides from about 9 to 14, and of magnesia from 3 to 9.

These "black granites," as will be seen by the foregoing description, are distinguished chemically from the ordinary granites by their low percentage of silica (45 to 67 per cent), their high maxima of iron oxides (9 to 14 per cent), and of magnesia (9 to 11 per cent), and mineralogical by their dominant feldspar not being a potash feldspar, and generally also by their considerable content of the darker iron-magnesia minerals.

Texture.

The general texture of the black granites corresponds in grade to that of the fine and medium granites.  In the diorites the arrangement and order of crystallization of the minerals always correspond to those of the granites, described on page 20.  In some of the gabbros this is also true, but in others and in diabase the arrangement greatly differs.  The feldspars are in needlelike crystals, between which the pyroxene has afterwards crystallized.

Physical Properties.

Aside from their great toughness, the diorites and the granitic gabbros probably differ but little in physical properties from granites of the same grade of texture.  By reason both of their peculiar texture and their mineralogical composition, the diabases and gabbros with "ophitic" texture, described on page 136, should differ considerably in physical properties from the granite.  As these stones are rarely used in large buildings, owing to the difficulty of quarrying them either in blocks of sufficient size or at low enough cost, data as to their compressive strength and other useful physical properties are not available.

The specific gravity of gabbro ranges from 2.66 to about 3, that of diabase from 2.7 to 2.98, and that of diorite averages 2.95.  In these rocks it thus usually exceeds that of granite.

As the black granites are used chiefly for monumental purposes, and particularly for inscriptions, their color, susceptibility to polish, and the amount of contrast between their cut or hammered and their polished surfaces are the physical properties of chief economic importance.

Doctor Merrill[41] explains the cause of these contrasts very satisfactorily:

The impact of the hammer breaks up the granules on the immediate surface, so that the light falling upon it is reflected, instead of absorbed, and the resultant effect upon the eye is that of whiteness.  The darker color of a polished surface is due merely to the fact that, through careful grinding, all these irregularities and reflecting surfaces are removed, the light penetrating the stone is absorbed, and the effect upon the eye is that of a more or less complete absence of light, or darkness.  Obviously, then, the more transparent the feldspars and the greater the abundance of dark minerals, the greater will be the contrast between hammered and polished surfaces.  This is a matter worthy of consideration in cases where it is wished, as in a monument, to have a polished die, surrounded by a margin of hammered work to give contrast.

The ordinary granites, while taking a high polish, do not afford such strong contrasts between hammered and polished surfaces as do the "black granites."  In some black granites this seems clearly to be due to their larger percentage of the black minerals, but in others, as some of the quartz diorites, in which the black minerals do not exceed those in some gray granites, the cause of this marked contrast much be sought in some optical property of the soda-lime feldspar and in its relative abundance.

"Black Granites" of Maine.

Classification.

The black granites at the quarries and prospects visited by the writer include:

(1)  Gabbros: Gabbro, hypersthene-olivine gabbro, norite, olivine norite.

(2)  An altered diabase porphyry.

(3)  Diorites:  Quartz diorite, mica-quartz diorite.

The appearance and the petrographic characteristics of the stone at each quarry will be stated in Part II of this bulletin, in the descriptions of the quarries and of their products, and a classification of black granites based upon economic principles will be found on page 75.  These black granites very considerably in shade and a little in color.  The olivine norite of the Heal quarry, near Belfast, is almost black, but under a side light shows small, brilliant dark-green areas of hypersthene.  The Vinalhaven olivine norite is quite black and is fine textured.  The Addison hypersthene-olivine gabbro is black, with small irregular white areas of feldspar.  The South Berwick gabbro is a very dark olive.  The Hermon Hill rock is porphyritic and dark green.  A little less dark than these, and without any greenish tinge, are the mica-quartz diorites of Beaver Lake (Calais) and the gabbros from Meddybemps Lake and Mingo Bailey's quarry, in Calais, while the quartz and mica-quartz diorites of Round Pond, East Sullivan, and Calais (Gardner) are all still lighter, and would pass for dark gray.  Most of these stones take a beautiful polish and all of them show very marked contrasts between the polished and cut surfaces.  That contrast naturally is still more marked in the darker ones.  The polished surfaces of most of these rocks show minute particles of magnetite.  Large blocks of Meddybemps Lake gabbro deflect the magnetic needle, and it is reported that the rock contains a very small amount of gold by assay, while platinum is reported from the Hermon Hill rock.

General Structure.

Rift.-The course of the rift at the black granite quarries is given in the quarry descriptions.  No other Maine black granite has such a marked rift as that of the Meddybemps Lake gabbro.

Sheets.-The sheet structure is not so well marked in the black granites as in ordinary granites, and herein lies the chief difficulty in quarrying them.  Pls. X, B, and XI, B, show the character of the sheets in the Round Pond quartz diorite. Pl. X, A, shows it in the Addison gabbro.  The sheets there range from 3 to 17 feet in thickness. In the Meddybemps Lake gabbro the sheets are well developed, being parallel to the banding and the rift, and are spaced from 1 to 6 feet.

Joints.-The courses of the joints are shown in the quarry diagrams.  Jointing in the Addison gabbro is shown in Pl. X, A, and in the Round Pond diorite its relation to a diabase dike is shown in Pl. XI, A.  Generally the spacing of the joints in the black granites is small, which prevents the quarrying of blocks of very large dimensions.

Variations in the Rock.

Banding.-The gabbro of Meddybemps Lake is traversed, in at least its upper part, by light-gray bands that range in thickness from one-fourth inch to 2 inches.  Their lighter shade is the result of a greater proportion of feldspar.  These bands dip at an angle of 15, and run parallel to both sheet and rift structure.  Pl. X, A, shows a similar but less pronounced banding in the Addison gabbro.

Pl. X-A.  Pleasant River Black-Granite Quarry, in Addison.  Looking north-northwest.  Showing the sheets crossed by frequent joints striking N. 80 E., the banding of the olivine gabbro, and several dikes of whitish quartz monzonite.

Plate X, A.

Pl. X-B.  Round Pond Black-Granite (Upper) Quarry, in Lincoln County.  Looking South-Southeast.  Showing the quartz-diorite sheets crossed by a 2 foot 4 inch dike of coarse pegmatite.

Plate X, B.

This banding represents not only the flow of the eruptive, but also different segregations of the principal minerals of the rock, alternating with one another.  This structure resembles that observed in certain Scotch gabbros, but in them the banding was also contorted prior to crystallization.[42]

Dikes.-The Addison gabbro is traversed by whitish dikes of fine-grained quartz monzonite, from 1 to 14 inches thick.  (See Pl. X, A.)  They have a border, from one-twentieth to three-fourth inch thick, of coarser material, in which the particles measure up to one-tenth inch in diameter.  The constituent minerals, in descending order of abundance, are soda-lime feldspar (oligoclase-andesine), potash feldspar (orthoclase) in slightly less amount, clear quartz, biotite, and hornblende, together with accessory magnetite, titanite, and apatite.  The texture of this dike rock differs from that of any of the aplite dikes examined from the granite quarries in that the soda-lime feldspar is mostly in lath-shaped crystals (0.37 by 0.07-0.11 mm.), although occasionally also in squarish forms, and makes up an irregular network, the meshes of which are filled with quartz.

In the mica-quartz diorite of the Beaver Lake quarry, near Redbeach (p. 164), there is a dike of grayish pinkish aplite, from 4 to 8 inches wide, which appears to be more recent than a neighboring dike of olivine basalt, as its branches cross it.  The particles in this aplite range from 0.11 to 0.91 mm., averaging roughly about 0.30 mm., and consist of soda-lime feldspar (oligoclase), much less potash feldspar, quartz, and biotite.  The aplite is thus a biotite granite.

Dikes of pegmatite and also of aplite traverse the Round Pond (Lincoln County) quartz diorite (see Pls. X, B, and XI, A), and pegmatites penetrate the overlying sedimentary schists.  Miss Bascom has described some schists and pegmatites from points in Johns Bay, about 8 miles south-southwest of Round Pond, which are probably of the same age.[43]  It is uncertain whether the lenses and dikes of pegmatite in the schists were formed prior to the veins in the diorite.

Pl. XI.  Round Pond Black-Granite (Lower) Quarry.

A.  Showing the quartz diorite traversed by a small dike of pegmatite, and both crossed by a 2 foot 6 inch diabase dike.  Looking west-southwest.  The diorite shows joints parallel to the dike. 

Plate XI, A.

B.  Southwest wall, showing a tongue of schist within the diorite, crossed by sheet structure; joint (B) at the right.

Plate XI, B.

The quartz diorite of Round Pond (Lincoln County) is also traversed by a diabase dike, which also crosses one of the pegmatite dikes and is therefore of later date.  (See Pl. XI, A.)  The mica-quartz diorite of Beaver Lake, near Redbeach, in Washington County, is traversed by a dike of olivine basalt, a very fine-grained black rock consisting of needlelike crystals of feldspar (andesine-labradorite) with pyroxene, olivine, and magnetite, together with accessory biotite and a white mica.

Contacts.-The only contact of black granite with other rocks well exposed at the quarries visited in the preparation of this report is at the Round Pond quarry.  (Peter Svenson & Co., p. 139.)  This has been referred to by Professor Wolff.[44]  At this place there is a finely plicated quartz-feldspar (andesine acid labradorite) hornblende-biotite schist, with accessory pyroxene, titanite, and apatite, striking N. 15 E., with numerous dikes of pegmatite, up to 2 feet 6 inches thick, parallel in places to the strike of the schist.  On the southwest wall of the lower quarry a tongue of this schist 10 to 15 feet thick and 40 feet long lies in the diorite, as shown in Pl. XI, B.  It reappears on the northeast wall.[45]  The contact of the diabase porphyry of Hermon Hill, near Bangor, is described on page 147.



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