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Excerpts From

The Granites of Maine, Bulletin 313

By T. Nelson Dale

(Continued)

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Joints.

Herrmann[26] divides joints into two groups-joints formed by lateral compression, whose distances from one another are related to the coarseness of the rock texture, and joints due to expansion, some of which are parted and filled with calcite, quartz, pegmatite, or volcanic rock.  That many joints are due to compressive or torsional strain, and that every such strain resolves itself into two components, resulting in two sets of joints that intersect at an angle of about 90, each forming an angle of about 45 with the direction of the strain, are facts now generally recognized.  Crosby[27] has suggested that torsional strains may have been supplemented by vibratory ones in causing joints.  Becker,[28] in a recent paper, shows that four or even more than four systems of joints may be due to a single force.  He also shows that subsequent strain on a region thus jointed would tend to produce motion along the previously formed joints rather than a new system of jointing.  It is conceivable that if a region had been jointed and afterwards subjected to a tensile strain, some of its joints might be parted, and if they were very deep the openings might become filled with volcanic matter from below, or, if not, with matter from above, infiltrated from overlying rocks.  That motion has occurred along some of the joints in the Maine quarries is evident from the polished and striated surfaces of the joints as well as from the faulting of the sheets.

The structural diagrams in Part II, accompanying descriptions of the quarries, show the course and dip of the joints at the Maine quarries.  The intersection of sheet structure by joint structure is shown in Pls. IV, A, B,; V. B; VI, A, B; VIII, A;, IX, A.  The conspicuous east-west system of joints as seen on Crotch Island is shown in Pl. II, B, and as seen in the region of Vinalhaven, on Heron Neck, at the south end of Green Island, in Pl. II, A.

Joints are exceptionally curved "as the side of a ship."  Thus at the White Granite Company's quarry, near Bluehill, there is a curved joint that covers a large segment of a circle and is continuous with two vertical joints, one of which strikes N. 50 W., and the other N. 52 E.  This, as well as the sheet structure intersected by it, is shown in Pl. VI, B.

Possibly related to such curved joints are what some New England quarrymen term "toe nails."  These joints strike with the sheets, but extend only from one sheet surface to the next, and have a curve which sharply intersects that of the sheet structure.  Such joints seem to be due to a strain different from that which produced the sheets.

A tabulation of 179 observations of joint courses made by compass at 80 Maine quarries of granite proper yielded the following results:

Courses of joints in granite as determined by 179 observations made at 80 quarries in Maine.
Direction. Number of observations.
N. 12 W.-N. 15 E 19
N. 20-30 E 13
N. 20-30 W 8
N. 32-50 E 25
N. 35-50 W 18
N. 55-70 E 19
N. 55-70 W 27
E.-W. to N. 75 E. and N. 75 W 50

The joints of the dominant system extend approximately east to west; those of the systems next most common extend approximately northeast and northwest, east-northeast, and west-northwest, and north and south.

The spacing of the joints varies considerably, ranging from 1 foot to 500 feet, but usually from 10 to 50 feet.

In some localities the jointing is very irregular.  Thus at the Ellis & Buswell (Ross) quarry, near Biddeford (fig. 39, p. 182), the granite is broken up into various polygons, which at the surface, where weather has made inroads, resemble bowlders.  Quarries opened in such places are called bowlder quarries.  Another sort of irregularity in joints consists in their discontinuity or intermittence, their strike and dip for the short spaces in which they occur being uniform.

Pl. VI-A.  Sands Quarry in Vinalhaven.  Looking S. 80 E.  Showing the curvature of the sheets, the intersecting joint face, and the N. 10 E. Channeling along the "cut-off."

Plate VI, A.


Pl. VI-B.  White Quarry in Bluehill.  Looking N. 10 W.  Showing the lenticular sheets crossed by a vertical joint curving from a N. 50 W. to N. 50 E. course.  The black vertical streaks are "underground water" issuing from between the sheets.

Plate VI, B.

 

Headings.

In some places joints occur within intervals so short as to break up the rock into useless blocks.  For a space of 5 to 50 feet the joints may be from 6 inches to 3 feet apart.  A group of close joints is called by quarrymen a "heading," possibly because, when practicable such a mass is left as the head or wall of the quarry.  Pl. VIII, A, shows a typical heading on Dix Island.  Pl. IX, B, from a photograph taken at the Longfellow quarry, near Hallowell, shows the intersection of two headings, one striking about northeast, the other about northwest; also the flexous course of the northwest set of joints.  Headings afford ample ingress for surface water, and consequently the granite within a heading is generally badly stained, if not decomposed.  This will be referred to more fully under the heading "Decomposition" (p. 54).

An interesting feature of both headings and joints shown in some of the deeper quarries at Quincy, Mass., which may be found in Maine as the quarries are deepened, is their vertical discontinuity.  A heading occurring at the surface may disappear below, or a heading may abruptly appear a hundred feet below the surface and continue downward.  Headings are not easily accounted for.  They may be produced by vibratory strains that recur at intervals of time.  If they are so caused, the character of the fractures in some headings indicate that the strains are very complex.

The courses of headings at each quarry are given in the descriptions of the quarries in Part II.

Faults.

The polished and grooved faces ("slickensides") observed on many of the joints at the quarries show that faulting has occurred along them.  The discontinuity of the sheets at some of the joints, causing, where the joints are slightly inclined, what quarrymen call "toeing in," may probably be attributed to faulting.  This supposition assumes, of course, that the sheet structure was formed prior to the jointing.  There seems to be good evidence of faulting on a considerable scale along the joints at Dodlin Hill, near Norridgewock, the details of which are described on page 150 and illustrated in fig. 33.  Faulting occurs also along sheets, displacing vertical flow structure, at the same quarry (p. 25), as well as displacing vertical dikes, as at the Allen quarry, on Mount Desert, as shown in Pl. VIII, B, and referred to on page 100.  The lateral faulting here has occurred both in northeast-southwest and in east-west directions.  Another faulted dike is mentioned on page 110.

Microscopic Fractures.

In some of the Maine quarries the granite near the surface acquires a marked foliation, which appears to be parallel to the sheet structure, and possibly to the rift.  This foliation is known by quarrymen as "shakes."  It occurs both at the top and at the bottom of the sheet, through a maximum thickness of 6 inches.  It is coextensive with the discoloration known as "sap" and occurs at many places near vertical joints.  Under the microscope this structure proves to consist of minute, nearly parallel fissures, of no great continuity, which traverse the mineral particles and which in the thin section examined are especially conspicuous in the quartz and the mica.  The distance between these fissures measures from a tenth to a half a millimeter, or from one two hundred and fiftieth to one-fiftieth inch.  The parallelism both to the sheets and the "sap" and its relation to the vertical joints indicates that the structure may be due to the freezing of surface water which has found its way to the sheets through the vertical joints and has entered the rift fissures.

The writer's attention was called to a similar structure in a quarry at Milford, N. H., consisting of short, parallel fractures along the rift, from one-half inch to 2 inches apart, having no apparent connection with joints or discoloration.  This is probably due to strain affecting part of the granite mass.

Subjoints.

Careful inspection shows that the joint structure in the Maine granites does not everywhere consist of a simple fracture, but that it is at many places complex.  Minute fractures branch off from the joint at an acute or right angle and penetrate the rock a few inches, or the rock for a few inches on either side of the joint is traversed by microscopic fissures that are roughly parallel to it.  All such structural features may properly be called subjoints.

A thin section of North Jay granite across a joint face shows two diverging subjoints that form an acute angle with each other and with the main joint and are filled with limonite and sericite (?).  Single subjoints are, however, rarely found, five or six fine parallel fissures generally occurring together.  In one of the quarries at Franklin (quarry of W. B. Blaisdell & Co., p. 94), the subjoints are parallel to the main joint, and as both main subjoints are filled with calcite, the granite near the joint weathers out vertically in small slablike pieces from one-half inch to2 inches thick, consisting of a central band of calcite, with one of granite on either side.  Under the microscope one of these subjoints, measuring 0.74 mm. across, is seen to be filled with long slivers of quartz and feldspar and scales of biotite, forming a breccia.  Another. 0.07 mm. wide, is filled with secondary quartz.  At the T. M. Blaisdell quarry in East Franklin, Hancock County (p. 93), a northeast-southwest vertical joint has on one side numerous subjoints that meander off at right angles to it and traverse a cubical mass whose sides measure 10 to 15 feet.  At the Shattuck Mountain quarry, in Calais (p. 164), a joint striking N 25 E. has subjoints striking 40 E., N. 60 E., and N. 50 W.

Woodworth has studied analogous and related structures in various rocks and has described them as "joint fringe" and "feather fractures."[29]

Contemporary Fractures.

Recent natural fractures have occurred, so far as known to the writer, at only three places in Maine granite quarries.  One of these fractures, already referred to in the discussion of the origin of sheet structure (p. 34; see also quarry description, p. 155), occurred at the Mount Waldo quarry, near Frankfort.  Here the course of the fractures ran from north-northwest to south-southeast through the center of the quarry for a distance between 200 and 300 feet.  The sheets removed from that part of the quarry aggregated about 20 feet in thickness.  The fracture was vertical and parallel to the flow structure, but at right angles to both sheets and rift and at an angle of 25 to the strike of the grain.  The other two fractures occurred at the Tayntor quarry, near Hallowell.  Their course was east-west.  One of these, which was 40 feet long and vertical, passed across a horizontal sheet 4 feet 6 inches thick, extending diagonally between two channel cuts that formed a right angle.  Here the rift is horizontal, a faint vertical grain structure strikes N. 70 W., and a vertical flow structure strikes N. 35 W.  At the Hooper, Havey & Company's quarry, in North Sullivan, the rock is under a compressive east-west strain, as it tends to fracture north-south across the grain and rift.  At many Maine quarries the horizontal movement of the rock crushes the "cores" left between adjacent drill holes in making "channels."

Pl. VII-A.  Lithonia (near), Georgia. Arch produced by the bursting of a thin sheet of granite-gneiss, Rock chapel Hill, near Lithonia, Ga.  Width of arch, 14 feet; height 9 inches.  Photograph by G. K. Gilbert, United States Geological Survey.

Plate VII, A.


Pl. VII-B.  Lower edge of schist inclusion in Granite on working face of Freeport Quarry.  Showing contact of schist and granite; also schist fragments more or less completely detached from the inclusion.  The black streak on the granite is ferruginous stain from the biotite schist.

Plate VII, B.

Rock Variations.

Under the term "rock variations" are grouped all those variations from typical granite that are due to injection, segregation, infiltration, inclusion, and steam cavities.

Dikes (Granitic).

The granitic dikes in the Maine quarries are of three kinds:  Extremely fine grained granite (aplite), very coarse grained granite (pegmatite), and fine or medium grained granite.

Aplite differs from the ordinary granite by the greater fineness of its texture and its scant content of mica.  It is known by quarrymen as "salt horse" or "white horse."  The courses of these dikes at each quarry are given in the diagrams or descriptions in Part II.  In thickness they range from a fraction of an inch to 6 feet, but usually from 2 inches to 2 feet.

The courses of 16 aplite dikes are distributed as follows:

Courses of 16 aplite dikes.
N.-N. 10 E. 3
N. 25 E. 1
N. 60-77 E. 2
N. 10-30 W. 4
N. 45 W. 1
N. 60-80 W. 5

Dikes that strike in the northwesterly-southeasterly quadrants are most numerous.

In color these dikes vary from bluish gray to light and dark reddish.  The texture of some aplites is so fine that the mineral particles can not be distinguished with the unaided eye; that of others is so coarse that the feldspar and mica may be thus detected.  Under the microscope the dimensions of the particles range from 0.05 to 0.75 mm., the average being about 0.16 mm. for the finer ones and 0.50 mm. for the coarser ones.  Some aplites have a porphyritic texture.

Two typical aplites will be described in detail.  One, from the John L. Goss quarry, on Moose Island, near Stonington, is from a dike 15 inches wide and over 200 feet long, consists largely of quartz, potash feldspar (microcline), and a soda-lime feldspar (oligoclase) in particles ranging from 0.047 to 0.141 mm. in diameter, a few thinly disseminated particles of the same minerals measuring from 0.55 to 1.45 mm. and a few scales of black mica measuring up to 0.47 mm.  Another aplite, from the Sands quarry, at Vinalhaven, consists mostly of quartz, but contains some potash feldspar (orthoclase and microcline), still less soda-lime feldspar (oligoclase), and a few scales of black mica.  The particles range from 0.047 to 0.3 mm. in diameter. 

The minerals of aplite dikes are so firmly attached to the granite on either side that the rock readily splits across both granite and aplite.  Under the microscope the minerals of the dike appear to be welded, so to speak, to those of the granite.  In construction the blocks containing such dikes should not therefore necessarily be regarded as places of weakness, but in a quarry at Franklin, Hancock County, the granite is close jointed for a space of a foot on either side of an aplite dike, the joints being parallel to the dike.

At the Bodwell Granite Company's quarry (see p. 168), 2 miles east of Jonesboro, Washington County, the reddish granite is traversed by a 6-foot dike of rather coarse, dark-reddish aplite, in which the higher power of the microscope shows that the source of the color lies in exceedingly minute dots of hematite.  The aplite contains also muscovite, biotite, and accessory pyrite.  This dike crosses the quarry in a N. 20 W. direction.  A similar dike, having a like course, but only 4 feet wide, occurs at the eastern end of the quarry.  A third dike, which ranges in width from 3 to 6 inches, has a N. 75-80 E. course, and a fourth, of fine-grained material, from one-half to 1 inch wide, crosses the others with a course N. 60 W. and can be traced for 200 to 300 feet.  This evidently of later date than the others.  Aplite dikes are supposed to have originated in the same deep-seated molten mass as the granite they traverse, but the represent a later state of igneous activity.  The fissures they fill were the result of various tensional strains or contractions, possibly, consequent upon the cooling of the granite.

Most aplites contain a slightly higher percentage of silica than granite.  Five analyses of aplites from the Far West made at the laboratory of the United States Geological Survey[30] show a range of silica from 71.62 to 76.03 per cent and an average of 74.08, which is near the maximum of silica for granites generally.

Pegmatite lies at the other extreme.  Its mineral constituents range usually from one-half inch to 1 foot or even several feet in diameter.  It is reported that the crystals in some pegmatite dikes measure from 10 to 30 feet in length by 1 to 3 feet in width.  The chief minerals in pegmatite dikes are the same as in granite, but they occur in different though varying proportions.  With these minerals are often associated tourmaline, garnet, beryl, etc.  Chemically these dikes generally contain more silica than the granite.  Dikes of pegmatite are, as a rule, more irregular in width than those of aplite.  They generally range in thickness from 1 inch to 10 feet.

 The courses of the pegmatite dikes in the Maine quarries and their relation to the structural features are shown in figs. 4, 6, 20, 21, 24, 28, 30, 31, and 33.  Their courses are distributed as follows in 15 quarries:

Courses of pegmatite dikes.
  Number
of quarries
N 3
N. 20 E. 1
N. 40 E. 1
N. 60 E. 1
N. 15-20 W. 3
N. 45 W. 1
N. 65-70 W. 2
N. 80-90 E. 2
Horizontal 1

Pl. X, B, shows a pegmatite dike crossing the diorite (black granite) at Round Pond, in Lincoln County.  At the Hallowell Granite Works (Longfellow) quarry ( p. 119) a 2-foot dike consists of milk-white soda-lime and potash feldspars (oligoclase and microcline), smoky quartz, biotite, and muscovite (black and white mica), and garnet.  The feldspars, quartz, and micas attain a length of several inches.  At the North Jay quarry (p. 82) the pegmatite dikes measure up to 2 feet 6 inches in width and consist of a milk-white potash feldspar, smoky quartz, biotite muscovite, the constituents measuring several inches in diameter.  At the Clark Island (J. C. Rodgers) quarry (p. 126) there are two intersecting pegmatite dikes with similar material of similar dimensions, together with black tourmaline and garnet.  The granite at Fryeburg, near the New Hampshire line, abounds in pegmatite.  At the Eagle gray granite quarry (p. 144 and fig 31) two dikes, one 5 feet, the other 10 feet thick, alternate with granite 25 and 60 feet thick.  The feldspar masses and crystals attain a length of 12 inches and the biotite and muscovite crystals and the quartz masses a length of 6 inches.  Small garnets are abundant.  Mingled with the pegmatite is some fine-grained aplitic material.  There is also considerable pegmatite at the Waldoboro quarry, in Lincoln County.  (See p. 142 and fig. 29.)  At the Wild Cat or Willard Point quarry of the Bodwell Granite company, now abandoned, there is a 12-inch pegmatite dike of feldspar, quartz, muscovite, and black tourmaline, which has a banded structure.  It is crossed by another dike half as thick, with a difference in strike of 20.

The origin of pegmatite has been much discussed both in Europe and in this country.[31]  The coarseness of its constituent minerals indicates slow crystallization, and the irregularity of the dikes shows tensional rather than torsional strain.  The banding of some pegmatite dikes and the isolated lenticular character of others indicate that the dikes were filled from heated solution, while many of them differ in no respect from dikes of igneous origin except by the coarseness of the texture.  For these reasons it is thought that pegmatite dikes in granite have been formed in openings and fissures that were due, possibly, to contraction while the granite was still hot and that some of these openings were filled with matter in a state of both molten plasticity and solution under pressure, and others by heated solutions that gathered matter from the adjacent granite.  Howsoever derived, this dike material crystallized very slowly. 

Pl. VIII-A.  High Isle Quarry, Muscle Ridge Plantation.  Looking east.  Showing sheets crossed by a N. 75 W. heading.

Plate VIII, A.


Pl. VIII-B.  Allen Quarry, west side of Somes Sound, Mount Desert, looking N. 15 W.  Showing thin lenticular sheets crossed by a vertical diabase dike, faulted on the fourth sheet from bottom of quarry; displacement, 16 inches along the sheet.

Plate VIII, B.

Granite.-Finally, there are dikes that differ from all those just described, formed simply of fine or medium-grained granite.  Thus at the Settlement quarry, near Stonington (see p. 108), the coarse granite is traversed by a dike, from 4 to 12 inches thick, of light inkish-gray granite, in which the feldspars attain a size of one-tenth of an inch (2.5 mm.), but under the microscope some are found that measure as little as 0.025 inch (0.12 mm.).  This rock consists of a pinkish potash feldspar (microcline), a white soda-lime feldspar (oligoclase-andesine), smoky quartz, and black mica (biotite).  At the Mosquito Mountain quarry (p. 153), near Frankfort, there is a 10-foot dike of medium-grained gray granite (quartz monzonite), with feldspars up to 0.3 inch.  The potash feldspar (microcline) is about equal in amount to the soda-lime feldspar (oligoclase), the quartz is smoky, and the mica is black.  At the Mount Waldo quarry (p. 155) there is a dike 200 feet wide of fine biotite granite, with coarse biotite granite on both sides of it.  The feldspars of this dike measure up to 0.15 inch, but range ordinarily from 0.36 to 1.45 mm.  The fine-grained biotite-muscovite granites quarried at the Sherwood quarry, on Crotch Island (spec. 25, a, described on p. 105), on East Bluehill (spec. 39, a, on p. 87), and at a small opening on Dodlin Hill, near Norridgewock (spec. 117, a, described on p. 152), all seem to belong to similar dikes that are not many feet in thickness.  At an old quarry near Bluehill (p. 85) there is an 18-inch dike of fine-grained muscovite-biotite granite, in which the feldspars are much intergrown with quartz.  The courses of these granite veins are N., N. 15 E., N. 20 W., N. 55 E., N. 70 W.  All such dikes represent granitic intrusions.

Veins.

Quartz veins are exceptional in the Maine quarries.  At the old Bodwell Company's quarry on Cook's Mountain, near Redbeach, now abandoned, the red granite is traversed by a banded grayish quartz vein, about 18 inches thick, that has a course N. 25 W. and a vertical dip.  It comprises three, or, in places, four bands, which differ mainly in texture and are separated by more or less pyrite in fine particles.  In places this vein divides into three smaller veins, each of which is from 3 to 4 inches thick.  The quartz contains some purple fluorite (lime fluoride), as determined by W. T. Schaller at the chemical laboratory of the United States Geological Survey, and a variable amount of a foliaceous lemon-colored mineral which Wirt Tassin, of the United States National Museum, has analyzed and determined as a new variety of sericite, resulting, possibly, from the alteration of a feldspar, and which is accompanied by another mineral, regarded by him as probably talc.  Mr. Tassin's analysis and report are as follows:

Analysis of yellow foliated mineral specimen of quartz marked "D. XXVI, 105a, '05."
SiO2 (silica) 53.28
Al2O3 (alumina) 23.06
Fe2O3 (ferric oxide) 0.10
FeO (ferrous oxide) 4.30
MgO (magnesia) 4.09
Na2O (soda) 0.65
K2O (potash) 8.90
H2O (water) 6.00

The mineral is secondary mica, probably derived from feldspar (although this is merely a conjecture), and will approximate sericite in composition.  It occurs in fine scales, occasionally compacted and then resembling serpentine.  Luster, pearly; color, greenish yellow; hardness, 2.5; specific gravity, 2.79 at 20 C.  It is associated in the vein with quartz, pyrite, purple fluorite, and another mineral which has a greasy luster and contains magnesia, but which it was impossible to separate in a state of sufficient purity for analysis.  This last mineral I believe to be talc.

Plate IX-A.  East corner of Waldoboro Quarry.  Showing contact between the granite in horizontal sheets and east-northeast dipping schist strata.

Plate IX, A.


Plate IX-B.  Northwest wall of Longfellow Quarry near Hallowell.  Showing intersection of two headings, one with a NW, the other with a N. 65 E. strike; also the progressive concentric ferruginous discoloration ("sap") from the sheet and joint surfaces.

Plate IX, B.


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