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The following discussion on granites was quoted from The Commercial Granites of Massachusetts, New Hampshire, and Rhode Island, published in 1908.

Definition of Granite

"Granite, in a general sense, is essentially an entirely crystalline igneous rock, consisting mainly of quartz, potash feldspar, and a feldspar containing both soda and lime, also of a small amount of either white or black mica or both, and sometimes of hornblende, more rarely of augite, or both. Where granite has, subsequent to its crystallization, been subjected to pressure sufficient to produce a parallelism in the arrangement of its minerals-that is, a schistosity-it is no longer a true granite, but a gneiss or granite gneiss; a sedimentary rock, however in becoming crystalline may resemble a granite gneiss and is called a sedimentary gneiss."

Economic Classification of Granite

Granite, for economic purposes, may be classified first as texture (even grained, coarse, medium, or fine). General color and shade is another classification of granite (for example, pinkish, reddish, lavender, gray or warm gray-"a gray showing the presence of a slight reddish, reddish-purple, or yellowish tinge"), or dark, medium, or light. Another classification would be by use-use in construction, monumental use, inscriptional use, polish, or statuary granites.

General Structure

Flow Structure

"Where two varieties of granite lie in contact, as at Redstone, N. H. . ., the dividing line between them indicates the direction of their flow. . . In some places this direction is also indicated by streaks or sheets of mica scales parallel to the direction of the line between the granites. . .The very local character of such structural features indicates that they are not due to pressure which affected the entire region, but that they originated while the granite masses were still plastic. A granite that exhibits flow structure is called by some writers a flow gneiss."

Rift and Grain

The rift in granite ". . .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. . . "

Sheets

Granite can be found divided into "sheets" or "beds" divided by joint-like fractures "which are variously curved or nearly horizontal, being generally parallel with the granite surface."

"The observations as to sheet structure, made at over 100 granite quarries in Maine and at many of those described in this bulletin, 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 area.

  2. The sheets increase in thickness more or less gradually downward.

  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, Maine, the sheet structure extends to a depth of at least 140 feet from the surface, and at Quincy to 175 feet.

  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 or to extend the sheet partings horizontally. . . "

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 the 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 consequent of the removal of the overlying material.

  4. To the 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 contraction under changes of temperature.

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

Joints

". . .joints can be 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."

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. . .Headings afford ample ingress for surface water, and consequently granite within a heading is generally badly stained, if not decomposed. . .An interesting feature of both headings and joints shown in some of the deeper quarries at Quincy, Mass. . . 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.

". . .They (the headings) may be produced by vibratory strains that recur at intervals of time. . . "

Microscopic Fractures

"In some quarries the granite near the surface acquires a marked foliation, which appears to be parallel to the sheet structure, and in places to the rift. This foliation is known by quarrymen as "shakes." It occurs both at the top and the bottom of the sheet, through a maximum thickness of six inches. It is coextensive with the discoloration known as "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."

Subjoints

"Careful inspection shows that the joint structure in 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 figures that are roughly parallel to it. All such structural features may properly be called subjoints."

Rock Variations

Dikes (Granitic)

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

"Aplite differs from 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."

"Aplite dikes are supposed to have originated in the same deep-seated molten mass as the granite they traverse, but they represent a later stage of igneous activity. The fissures they fill were the result of various tensional strains or contractions, possibly consequent upon the cooling of the granite.

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. . . "

"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. . .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.

"Granites - Finally, there are dikes that differ from all those just described, formed simply of fine or medium-grained granite."

Veins

"Quartz veins occur rarely in the Maine granite quarries. . .and at but few of the quarries here. . .Some of these seem to be clearly of pegmatitic origin."

Dikes (Basic)

"Dikes of dark-greenish or black, hard and dense rock (diabase, rarely basalt) are of very common occurrence in the Maine and Massachusetts quarries. . .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.

"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, can not 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. . .They are finer grained than granite. . .The cause of knots is not perfectly understood. They are collections (segregations) generally 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 have been found by the writer at the Jonesboro and Marshfield quarries in Maine and at Redstone, N. H. Their greatest diameter was about 12 inches, and the lining crystals were quartz, more or less amethystine and feldspar. . .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 and 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. In the Maine quarries they range from an inch to 40 feet in length.

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. . .Generally, however, the discoloration diminishes gradually from without inward. In some quarries there seems to be a connection between the "shake" structure 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 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."

"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, possibly allanite. Such stains usually. . .arise from pyrite particles or crystals."

"Another kind of discoloration occurs on either side of diabase or basalt dikes, caused mainly by various alterations of the feldspars and their consequent change in shade or color."


Source:

Dale, T. Nelson, The Chief Commercial granites of Massachusetts, New Hampshire and Rhode Island, Bulletin 354. Department of the Interior, United States Geological Survey, Washington Government Printing Office, 1908.



Discussion of Granites 1908

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