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Excerpts From

The Granites of Maine, Bulletin 313

By T. Nelson Dale

(Continued)

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Maine Granites.

Classification.

The granites exposed at the Maine quarries fall naturally into six groups:

1. Biotite granite, consisting of the two feldspars, quartz, and black mica.
2. Muscovite-biotite or biotite-muscovite granite, with both black and white mica, the name of the predominating mica being in each case the first.
3. Hornblende-biotite or biotite-hornblende granite, with hornblende and black mica, named according to the predominance of one or the other of these minerals.
4. Quartz monzonite, in which the amount of lime-soda feldspar is so large as to about equal that of the potash feldspar.  The monzonites quarried in Maine contain biotite or biotite and hornblende.
5. Hornblende granite, consisting of the feldspars, quartz, and hornblende.
6. Quartz diorite, used for building purposes and not classed commercially as "black granite."  This contains only lime-soda feldspar, with quartz, hornblende, and biotite.

The first three groups include nearly all the granite quarried in the State.  Quartz monzonite is quarried at Sprucehead, Knox County, and at Norridgewock, Somerset County; hornblende granite is quarried in a small way on Mount Desert Island, and quartz diorite is quarried at Bryant Pond, Oxford County, and has been quarried at Hartland, Somerset County, and for local use at Alfred, York County.

The general appearance and petrographic peculiarities of the stone at each quarry will be briefly stated in Part II, in the descriptions of the quarries and their products,[1] and a classification of Maine granites based upon economic principles will be found on pages 72-75.

Maine granites as exposed at the quarries show a wide range of texture.  Some are porphyritic, others even grained, ranging from very fine, in which the size of the particles averages about one-fiftieth inch (one-half millimeter) to very coarse, in which the feldspars measure an inch or more in diameter.   They exhibit also considerable variety of color-pinkish, reddish, gray of various shades, and light lavender.  The differences in the color of the two feldspars and the variations in the amount of biotite and in the size of its scales produce more or less marked contrasts of color and of shade.  The quartz also, if smoky, darkens the general color, and if clear, lightens it.

General Structure.

The term "structure" embraces all the divisional planes that traverse the rock.  These occur at intervals ranging from a microscopic distance to one measured by scores of feet, and either cross or, very rarely, give a course to the texture resulting from crystallization.

Flow Structure.

At some of the quarries (as Dodlin Hill, near Norridgewock, and Clinton Sherwood quarry, on Crotch Island) two varieties of granite lie in contact, the dividing line between them being vertical (see p. 109 and figs. 18 and 32 for details).  One of the granites at Dodlin Hill also shows a light and dark vertical banding.  The direction of the flow of the granite at these quarries must therefore have been vertical.  At the Mount Waldo quarry, near Frankfort (see p. 154), vertical flow structure also occurs.  At Tayntor & Company's quarry, near Hallowell (see p. 120), a faint vertical banding is visible in one of the walls, and a thin section of the rock shows a parallelism of the biotite.  The same parallelism is seen also at an old quarry near Brunswick (see p. 76).  The arrangement of the mica in the granite at both places was doubtless governed by the direction of the flow of the material prior to its crystallization.  At one of the North Jay quarries (see p. 82) a similar parallelism in the mica occurs, but its course is in horizontal waves 20 feet wide and 3 feet high; while at another of these quarries similar waves pitch 10-40.  At the Pownal Granite Company's quarry, in Pownal (see p. 79), the rock shows a parallelism of the minerals, the planes of structure dipping 10, and a thin section of the granite does not show any bending of the mica plates or straining of the quartz particles.

In some of the Massachusetts and New Hampshire quarries the writer found flow structure parallel to the surface of the granite at its contact with the overlying rock and also surrounding and parallel to the surface of large blocks of other rock included with the granite.  (See "Inclusions," p. 52.)

The very local character of these structural features indicates that they are not due to pressure which affected the entire region, but that they originated while the granitic masses were still plastic, for they conform to the general direction of the flow or to some local modification of it.  A granite that exhibits flow structure is by some writers called a flow gneiss.  The courses of the lines of this flow structure in the Maine quarries, when the bands are vertical, are N. 35 W., N. 20 W., and N. 45 E.

Rift and Grain.

The rift in granite is a feature of considerable scientific interest and of much economic importance.  It is an obscure microscopic foliation-either vertical, or very nearly so, or horizontal-along which the granite splits more easily than in any other direction.  The grain is a foliation in a direction at right angles to this, along which the rock splits with a facility second only to that of the fracture along the rift.  After a little experience an observer can detect the rift with the unaided eye, where it is marked.  The only available data on this subject are furnished by Tarr and Whittle.[2]

Tarr presents four figures reproduced from drawings made from enlarged views of thin sections showing the rift in Cape Ann hornblende-biotite granite.  These figures and his descriptions indicate that rift consists of microscopic faults, most of which meander across feldspar and quartz alike, although some go around the quartz particles rather than through them.  In the feldspars rift usually follows the cleavage.  These minute faults are lined with microscopic fragments of the mineral they traverse and some of them send off short, minute diagonal fractures on either side.  In examining such a structure it is important to make sure that the grinding of the section has not in any way modified the original fractures.  Tarr adds that at Cape Ann the rift does not traverse the "knots" or the basic dikes that cross the granite.

Whittle gives two sketches made from polished surfaces of a well-known granite quarried by the Maine and New Hampshire Granite Company at Redstone, N. H.  One of these sketches made from a surface running at right angles to the rift, shows quartz and feldspar grains traversed by a generally parallel set of lines corresponding to the rift planes.  The lines are more numerous in the feldspar than in the quartz grains.  The other sketch, made from another specimen, shows besides the rift lines another less pronounced set intersecting these at right angles.  This second set corresponds to the grain.  Whittle calls attention to the fact that notwithstanding the marked rift and grain at this quarry the stone stood a compression test of 22,370 pounds to the square inch, and was, therefore, not appreciably weakened by the microscopic fractures.  A visit made by the writer in 1906 to the quarry at Redstone, N. H., has corroborated Whittle's observations.  The details of the rift and grain structure observed there will be discussed in a future publication.

Another peculiarity of rift is that the angle of its inclination may at some places be modified by gravity.  Thus in some localities a block will split at one angle from the top, but at another from the side; or, again, at one angle where the mass of the block is at the right and at another where it is at the left of the line of fracture.  Experienced granite workmen at Concord, N. H., and Quincy, Mass., report that at some places a block that would show a horizontal rift when split from one point of the compass (say the north) acquires an inclined rift if split from the south or the east or west.  The cause of this is not apparent.  There are also indications that a slight alteration of the feldspars may improve the rift.  Finally, as is well known to granite quarrymen, rift and grain are modified by temperature, the effect of winter cold in New England (frost?) being to intensify the rift and grain where they are weak.

A Norwegian geologist, Carl C. Riiber, in a work on the granite industry of Norway[3] describes an augite syenite with inferior rift and grain, in which the cleavage planes of the individual feldspar crystals are parallel to the two cleavage planes of the rock.  No such relationship between rift and mineral cleavage has yet been made out in the Maine granites.

Among the many thin sections prepared for this bulletin there is one from the medium-grained biotite-muscovite granite of C. E. Hudson's Weskeag quarry, near Pleasant Beach, South Thomaston, which shows the rift; and this is also quite marked in the hand specimen.  It consists of exceedingly delicate fractures that meander across the quartz particles and some of the feldspars in a roughly parallel direction.  These cracks are filled with a highly refracting mineral (calcite or muscovite?), showing that the fractures are not recent.  (See fig. 1.)

Hermann[4] states that in Saxony the rift is parallel to the horizontal sheets or joints.  That is true for short distances in the Maine quarries, but where the rift is horizontal and the sheets curve it crosses the sheets, and of course where the rift is vertical it crosses them throughout.  Exceptionally, in one of the quarries at Quincy, Mass., a foreman reported to the writer a deflection of the rift in apparent relation to the increasing inclination of the sheets.  Pl. II, A, shows the relations of the rift to the sheets at the Ryan-Parker quarry on Crotch Island.  The structural diagrams accompanying the quarry descriptions show the relation of the rift of grain, when vertical, to the various joints.

Rift and grain data were collected at 53 quarries of granite proper.  At 29 of these the rift was vertical and at 24 it was horizontal and the grain was vertical.  The courses of the rift and grain are distributed as follows:

Courses of rift.
  Number of quarries.
N. 10 W.-N. E 6
N. 22-50 W 5
N. 30-77 E 3
N. 60-70 W 9
E.-W. to N. 85 E. and N. 80 W 6
N 2
N. 20-25 W 2
N. 45-75 E 4
N. 45-72 W 6
E.-W. to N. 80 E 10

It appears, therefore, that when rift or grain is vertical the east-west and west-northwest to northwest courses are the most common, and next the north and east-northeast to northeast courses.

Rift and grain are not always pronounced.  Either or both may be very feeble or may be absent.  At some of the Redbeach quarries, owing to the absence of both, it is difficult to hammer out even a good hand specimen.

At the Armbrust quarry, in Vinalhaven, there is a horizontal rift confined to a 4-foot mass striking across the hill with a N. 65 W. course.

The presence of fairly good rift or grain is an important economic factor in the granite industry, for it diminishes both the amount of labor in drilling for blasts and in splitting.

The cause of rift structure and the relative time of its formation are not yet known.  If rift were always vertical it might appear to be closely related to those joints which are nearly parallel to it; but that would not explain the rift when horizontal, and horizontal rift can not be related to the sheets, which it intersects at various angles in many granites.  In general it is evident that since it crystallized granite has been subjected to strains that have caused either two sets of vertical microscopic fractures extending at right angles to each other, one more pronounced than the other, or one set of similar horizontal fractures crossed by a vertical set.

That the rift is a factor in the crushing strength of granite is shown by the results of tests of Mount Waldo granite from Frankfort, blocks of which when placed on the bed that is, with pressure applied at right angles to the rift-showed an ultimate strength to the square inch of 31,782 to 32,635 pounds (average, 32,208), but when placed on the side-that is, with pressure applied parallel to the rift-showed an ultimate strength of from 29,183 to 30,197 pounds (average, 29,690).  In the first test the first crack appeared in the block at a pressure of from 120,000 to 123,300 pounds (average, 121,650) and in the second test it appeared at one of 107,400 to 112,600 pounds (average, 110,000 pounds).[5]

Sheets.

The division of granite into "sheets" or "beds" by jointlike fractures which are variously curved or approach horizontally, being generally parallel with the granite surface, attracted the attention of geologists long ago.  Although this is the most striking feature in every granite quarry and largely makes the granite industry possible, there is a great diversity of opinion as to its cause.  Whitney[6] writes:

The curves are arranged strictly with reference to the surface of the masses of rock, showing clearly that they must have been produced by the contraction of the material while cooling or solidifying, and also giving very strongly the impression that, in many places, we see something of the original shape of the surface as it was when the granitic mass assumed its present position.

Shaler, a few years later,[7] attributed the sheet structure to expansion due to solar heat.

C. H. Hitchcock[8] notices in New Hampshire granite "numerous joints, the planes of which correspond very nearly with the slope of the hill,: but does not undertake to explain them.

Vogt[9] states that the sheets in granites in southeastern Norway measure from 6 inches to 6 feet in thickness and dip from 8 to 33 on the sides of the mountains, toward the valleys, but that they are horizontal on top and approximately parallel to the surface.  He shows that they are of preglacial origin, attributes them to the same cause that is postulated by Whitney for those in California, and regards them as parallel to the original surface of the granite masses.

Harris,[10] referring to the English granite quarries, writes:  "In every quarry we visited we found that the direction of the 'beds' approximately correspond with the outline of the hill on which it was situated."  He offers no explanation of the phenomenon, however.

J. J. Crawford[11] describes the sheet structure at granite quarries in Madera and Tulare counties, California, as consisting of "concentric layers conforming in a general way to the contour of the hills," but he suggests no cause for them.

Hermann,[12] who made a special economic study of the granites of Saxony, writes:

Upon closer inspection it appears that the granite sheets are elongated lenses overlying one another, of which the upper one, as a rule, has its bulging part lying in the depression formed by the two underlying lenses where they come together.

 Branner[13] describes the exfoliation of the granitoid gneisses in Brazil, which he attributes only in part to changes of temperature.  He calls attention to the fact that the linear expansion of a mass of gneiss 300 feet long at a depth of 15 feet from the surface under a surface temperature of 103 F. would amount to only 0.072 inch; and he quotes the results of Forbes, Quetelet, and others to show that the annual change of temperature can penetrate rock only to a depth of 40 feet in temperate regions and still less in the Tropics.

Merrill[14] describes Stone Mountain, in Georgia, as a boss of granite 2 miles long by 1 1/2 miles wide and 650 feet high, which owes its form wholly to exfoliation parallel to preexisting lines of weakness.  The mass appears to be made up of imbricated sheets of granite which he regards as the result of torsional strains.  The bosslike form is incidental and consequent.  Intermittent expansion and contraction from changes of temperature have so affected the sheets that bound the mass at the sides that they have found relief in expansion in an upward direction.  These ruptured sheets are rarely more than 10 inches thick, but are 10 or 20 feet in diameter.[15]

Hermann[16] sums up his conclusions on the subject substantially as follows:  The so-called sheets are thin near the rock surface, generally only a few centimeters thick, but become gradually thicker with increasing depth.  This downward increase in the thickness of the sheets is generally more rapid where the texture of the stone is coarser.  The course of the sheets is not, as Vogt claims, parallel to the original surface of the consolidating rock.  It is not governed by internal strains.  The attitude of the sheets corresponds to the form at the present rock surface.  The sheet structure is to be looked upon as the effect of the beginning and progress of weathering from the surface inward.  These weathering cracks are determined by the form of the rock surface instead of that being determined by them.

Turner[17] calls attention to the sheet structure and exfoliation of Fairview dome in Yosemite.

Gilbert[18] shows that sheet structure occurs in synclinal as well as in anticlinal attitude-in other words, is parallel to hollows as well as hills-which he considers unfavorable to the theory that it is an original structure.  He suggests that sheet structure may possibly be due to expansive stress consequent upon relief from compressive stress brought about by the removal of the mass into which the granite was intruded.  Subordinately he notes that in the Sierras, at least, the dome structure and the parallel joint structure do not occur in the same place and that the former has resisted general erosion more successfully than the latter.

Dr. G. F. Becker, in a recent conversation with the writer, stated that he had found the granites and gneisses at the bottom of the Colorado Canyon both vertically and horizontally jointed.  If these are true granites and are still in contact with the rocks into which they were intruded and show genuine sheet structure the phenomenon would conclusively prove that such structure may occur independently of solar heat and load.

Mr. S. F. Emmons similarly stated that in the Mosquito (Park) Range, in Colorado, the pre-Cambrian granite and schist are cut by horizontal joints to a depth of 50 feet below their contact with the overlying Cambrian, the joints diminishing in number downward.  The original load upon the granite here consisted of at least 10,000 feet of Paleozoic and between 5,000 and 6,000 feet of Cretaceous rocks.  As the granite, however, was not intruded into Cambrian sediments it must have been exposed to atmospheric erosion before they were deposited.  These horizontal joints may therefore have been related to solar temperature.

Mr. G. K. Gilbert has recently studied the granite domes of Georgia and attributes their sheet structure to compressive strains.  He finds that the granite in these domes[19] is not naturally divided into plates, but that the outer parts of the granite-the parts nearest the surface-are in a condition of compressive strain, which results in slow exfoliation and which enables quarrymen, by means of carefully regulated blasts, to develop joints that run approximately parallel to the surface, so that the granite is detached in sheets.  As these sheets are divided into blocks in the process of quarrying the blocks expand horizontally as they are released from the general mass.  In these granitic domes parting planes also develop naturally within a few inches of the surface, and the expansive force is there so great as to induce conspicuous buckling in the thin sheets thus formed.  This buckling is illustrated in Pl. VII, A, from a photograph taken by Mr. Gilbert on Rock Chapel Hill, near Lithonia.  The jar of blasting precipitates this sheeting action, so that several of the domes at which quarrying is in progress show long lines of freshly formed disrupted arches.  Mr. Gilbert found that the horizontal elongation, or rather the elongation coincident approximately with the contour of the dome surface, amounted, by one measurement, to three-fourths inch in a length of 40 feet.

The artificial production of sheets in granite, as practiced at Bangalore, in southern India, shows similar phenomena.  It is described by H. Warth[20] in a substance as follows:  At the surface there is a horizontal sheet of rather weathered rock 4 feet thick; below this lies a sheet of fresh rock 3 feet thick, but below this lies fresh rock without split.  These sheets "are probably due to the variations of temperature, daily and seasonal."  By means of wood fires plates 60 by 40 feet by 6 inches in thickness are detached in one piece.  A line of fire 7 feet long is gradually elongated and moved over the granite.  The effect of the fire is tested by hammering the granite in front of it, and then the fire is moved forward.  The maximum length of the arc of fire is 25 feet.  The burning lasts eight hours; the line of fire is advanced 6 feet per hour.  The area passed over by line of fire is 460 square feet.  The amount of wood used is 15 hundredweight.  The average thickness of stone is 5 inches and its specific gravity is 2.62.  These data show that 30 pounds of stone are quarried with 1 pound of wood.  Some plates are taken out in inclined position.  The action of fire is independent of the original surface of rock and also of the direction of lamination (the granite is gneissose) and of veins.  The uniformity in the thickness of the sheets is attributed to the regulating influence of preexisting cracks.

Van Hise,[21] in his treatise on metamorphism, is inclined to attribute sheet structure to solar temperature.

Before these various views are discussed the sheet structure as exposed at the Maine quarries will be described.

Dome form and sheet structure are most finely exhibited at Crotch Island, near Stonington, and at Mosquito Mountain, near Frankfort.  Pl. II, B, shows the structure in the southern half of Crotch Island, at Thurlow Head.  The dome is oblate, measuring about 1,500 feet from north to south and 140 feet in height. 

Plate III, A, from a photograph of the Ryan-Parker quarry, at the southern edge of the dome, shows that the sheets rapidly increase in thickness downward-from 1 to 25 feet in a depth of 75 feet-and that they dip 20-25 south.  At the next quarry north, the Goss quarry (see p. 104), the excavation has exposed the center of the dome mass.  Here the sheets dip both north and south, measure from 1 to 30 feet in thickness, and extend to a depth of fully 140 feet from the surface.

Mosquito Mountain, 2 miles south of Frankfort, in Waldo County, is an oval granite dome 545 feet high, with a north-south axis about 1 mile long and measuring about half a mile across.[22]  It has a steep east face, the sheet structure of which is shown in Pl. III, B.  On the top of this mountain, where the quarry is situated, the sheets dip gently north, west, and east, tapering out on the sides, and measure from 6 to 15 feet in thickness.  At the Mount Waldo quarry, which lies 1 1/4 miles north-northwest of the top of Mosquito Mountain, the sheets dip 10 and measure from 8 inches to 8 feet in thickness, and the excavation averages about 20 feet in depth, about 300 feet from north to south, and 400 feet from east to west.  The granite here is evidently under compressive strain, for the progress of quarrying resulted in a vertical fissure, running north-northwest by south-southeast for the entire width of the quarry and across the rift, which is horizontal.  The formation of the fissure was accompanied by a dull explosive noise.  At several other quarries in the State foremen report a partial closing of vertical drill holes by expansion or compressive strain of the rock.  (See pp. 121, 142.)

At the White quarry, in Bluehill, the granite breaks with explosive sound when split in large sheets along a vertical rift that extends N. 50 W.  The gradual increase in the thickness of sheets downward is well shown at the Stinchfield quarry, near Hallowell (Pl. IV, B).  Their evenness and curvature are shown at the Sands quarry, at Vinalhaven (Pl. VI, A).  At the Hurricane Island quarry (Pl. IV, A) the excavation is 105 feet deep.  The upper sheets measure from 3 to 20 feet in thickness, but the lowest sheet is fully 60 feet thick.  A good cross section of granite sheets is seen at the Crabtree & Havey quarry, in Sullivan, shown in Pl. V, B, which brings out their lenticular form and arrangement.  The tapering end of one lens lies between the thickest parts of two others.  This accounts for the apparent irregularity in their thickness of the sheets in some longitudinal sections, notwithstanding their progressive thickening downward.  Compare Pl. V, A, taken at the same quarry, with Pl. V, B.  Pl. VIII, B, also shows the tapering of the sheets, but here there has been some faulting since their formation, as is shown by the discoloration of the dike.

Faulting of the sheets is likely to occur also along the steep joints.  (See also p. 98.)  Pl. IX, A, a view taken at the Waldoboro quarry, shows the relation of the sheet structure to the underside of the originally overlying mass of schist, a remnant of which bounds the quarry on two sides at the top.  The sheets here are nearly horizontal, while the schist dips 45.

The observations as to sheet structure made at over 100 granite quarries in Maine are here summarized:

  1. There is a general parallelism between the sheets and the rock surface, resulting in a wavelike joint structure and surface over large areas.

  2. The sheets increase in thickness more or less gradually downward.  In the coarse-textured granites of Crotch and Hurricane Islands the increase is abrupt.  (See Pls. III, A, and IV, A.)

  3. The sheets are generally lenses, though in some places their form is obscure.  Their thick and thin parts alternate vertically with one another.  The joints that separate these superposed lenses therefore undulate in such a way that only every other set is parallel.

  4. On Crotch Island the sheet structure extends to a depth of at least 140 feet from the surface.

  5. There are indications here and there that the granite is under compressive strain, which tends to form vertical fissures or to expand the sheets horizontally so as to fill up small artificial fissures.  (See p. 34.)

The observations made in Europe and in this country, taken in connection with the various inferences geologists have drawn from them, indicate that sheet or "onion" structure in granite rocks is due to the following possible causes:

  1. To expansion caused by solar heat after exposure of the granite by erosion.

  2. To contraction in the cooling of the granite while it was still under its load of sedimentary beds, the sheets being therefore approximately parallel to the original contact surface of the intrusive.

  3. To expansive stress or tensile strain brought about by the diminution of the compressive stress in consequence of the removal of the overlying material.

  4. To concentric weathering due to original texture or mineral composition.  This action would be chiefly chemical and would be aided by vertical joints and by any superficial cracks due to expansion and to contraction under changes of temperature.

  5. To compressive strain akin to that which has operated in the folding of sedimentary beds.

  6. To the cause named under 1 at the surface, but to the cause named under 5 lower down.

These propositions will be considered in the order given:

  1. Solar heat may produce a certain amount of exfoliation in thin sheets at the surface, as is proved experimentally in the fire method of granite quarrying in India (p. 33), but as it penetrates only to a depth of 40 feet and as sheet structure is known to occur on Crotch Island, Maine, at a depth of 140 feet and at Quincy, Mass., at a depth of 175 feet, it is quite inadequate to account for sheets that are 20 to 30 feet thick and 100 to 175 feet below the surface.

  2. In view of the load under which granite was probably formed, as shown by the well-known calculations of Sorby and Ward,[23] and the very gradual rate at which, therefore, it probably cooled, which is also indicated by the general coarseness of its texture, it is very improbable that the temperature at its contact surface and the temperature at depths 100 or 200 feet below could have so greatly differed as to bring about such a system of joints by contraction.

  3. As Gilbert states, in suggesting the theory of fracture by relief to tensile strain through the erosion of overlying masses, we have no distinct knowledge of it.  It is a possible explanation.

  4. Careful inspection of the rock on both sides of the sheet joints fails to show any difference in texture or mineral composition.  The sheet structure traverses both rift and flow structure, and it would be possible to procure specimens showing a sheet joint traversing a single crystal of feldspar.  Whatever chemical action has taken place along the sheet joints is of secondary character.  Acid waters may have gained access to the joint, but have not caused it.  (See matter under heading Discoloration, "Sap," etc., p. 52.)

  5. The condition of strain described by Merrill and Gilbert as existing in the granite domes of Georgia and by Niles and Emerson in the gneiss at Monson, Mass.,[24] and occurring to a lesser extent in some Maine quarries (see pp. 93, 112, 121, 155), shows that granite and gneiss are in places still under compressive strain.  Another instance occurs at the quarry of the New England Granite Works, at Concord, N. H., recently visited by the writer.  The foreman at this quarry was in the habit of calling certain sheets, marked by the absence of discoloration, "strain sheets," to distinguish them from the others.  At one place a northwest-southeast compressive strain had actually extended the strain sheet about 5 feet, and also caused a vertical fracture that extended over 15 feet diagonally from the north-south working face to a point on a vertical east-west channel 5 feet back of the face, closing up the channel to half its original width.  The practicability of developing sheet structure by use of explosives and compressed air, as it is developed in some of the North Carolina granite quarries, shows that the rock is under compressive strain there.[25]

    All these observations bring this theory within the domain of inductive science.  If sheet structure is due to compressive strain, it is due to such a strain as would produce a series of undulating fractures extending entirely across a granite mass several miles in diameter and to a depth, as far as observed, of 175 feet from the rock surface.

  6. In view of the undoubted sheeting effect of expansion under solar hit within a short distance of the surface and of the fact that some of the sheets near the surface measure but a few inches in thickness, it is quite possible that very thin surface sheets have originated in this way; but in view of what was stated under 5 it seems rather probable that compressive strain is the main factor in producing massive sheets.  At the surface both causes may have cooperated.  The progressive thickness of the sheets downward indicates that the operation of this strain is evidently also dependent upon distance either from the present surface or from a former surface or contact.

According to this view sheet structure may be said to exert a controlling influence upon surface forms, yet it seems quite admissible that granite domes as conspicuous as Stone Mountain, in Georgia, and Fairview Dome, in California, notwithstanding all the exfoliation that has taken place on them or the erosion they may have suffered, may still retain some degree of parallelism between their present form and the original contour of the granitic intrusions of which they are parts.  This may be true, also, of the granite hills of Mount Desert.

The probability being admitted that the general parallelism between the present surface and the sheet structure is the result of erosion that followed the sheeting, the question still remains, What has determined the form and location of the domes?  These may possibly be referred to major arches (anticlines) in the folds of the stratified rocks which originally overlay the granite.  The crustal movement that produced these folds may also have brought about the intrusion of the material that formed the domes beneath them.

Although the sheet structure and the rock surface are very generally parallel, they are not universally so, as may be seen on the west flank of Mosquito Mountain, shown in Pl. III, B, which has evidently been partially eroded, and at the Clark Island (p. 126) and Sprucehead (p. 124) quarries, where the rock surface and the sheet structure were also found to be discordant.

Sheet structure in granite so much resembles the structure of folded stratified rocks that underground water circulates in practically the same way along the fracture planes of one and bedding planes of the other.  The exudation of water along sheet joints on vertical rock faces is seen in many of the Maine quarries, and is shown in Pl. VI, B.

That sheet structure is not confined to intrusives is shown at quarries in Niantic, R. I., and Milford, N. H., where it passes indifferently from the granite into the overlying gneiss.

PL. V.  Crabtree & Havey Quarry in Sullivan 

A.  South side, showing irregularity in thickness of sheets owing to their lenticular form; also 9 black knots.  The cuts are along grain and hard way.

Plate V, A.

B.  East wall, showing lenticular form of sheets in cross section on a joint face.

Plate V, B.



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