. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. THE GNEISSES 143 gneisses is essentially the same as that of the granites, from which they differ only in structure and origin. They, how- ever, present a greater variety and abundance of accessory minerals, chief among which may be mentioned (besides those of the mica, hornblende, or pyroxene group) garnet, tourmaline, beryl, sphene, apatite, zircon, cordierite, pyrite, and graphite. Structure. — Structurally the gneisses are holocrystalline gran- ular rocks, as are the granites, but they differ in that the various. -O' Fig.
. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. THE GNEISSES 143 gneisses is essentially the same as that of the granites, from which they differ only in structure and origin. They, how- ever, present a greater variety and abundance of accessory minerals, chief among which may be mentioned (besides those of the mica, hornblende, or pyroxene group) garnet, tourmaline, beryl, sphene, apatite, zircon, cordierite, pyrite, and graphite. Structure. — Structurally the gneisses are holocrystalline gran- ular rocks, as are the granites, but they differ in that the various. -O' Fig. 15.—Microstructure of gneiss, showing at the point a broken feldspars. constituents are arranged in approximately parallel bands or layers, as shown in PL 12. In width and texture these bands vary indefinitely. It is common to find bands of coarsely crystalline quartz several inches in width, alternating with others of feldspar, or feld- spar, quartz, and mica, or hornblende. A lenticular structure is common, produced by lens-shaped aggregates of quartz or feldspar, about and around which are bent the hornblendes or mica lamince. The rocks vary from finely and evenly fissile through all grades of coarseness, and become at time so mas- sive as to be indistinguishable in the hand specimens from granixes. Colors.—Like the granites, the gneisses are all shades of gray, greenish, pink, or red. Geological Age and Mode of Occurrence. — The true gneisses are among the oldest crystalline rocks, and have been considered. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Merrill, George P[erkins] 1854-. New York, London, The Macmillan company
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