Structural geology . minerals of this kind are uniformly idio-morphic or porphyritic in cleavable rocks. They develop byrecrystallization after rock flowage has ceased, but probablywhile the rock is still under high pressure and temperature, asis evidenced by their high specific gravity and characteristic 1 Adams, F. D., Report on the geology of a portion of the Laurentian area lyingto the north of the island of Montreal: Ann. Rept. Geol. Survey of Canada, Vol. 8,pt. J, 1896, p. 85 et seq. PORPHYRITIC TEXTURE 91 occurrence in the proximity of intrusive igneous rocks. Theirlate development by r
Structural geology . minerals of this kind are uniformly idio-morphic or porphyritic in cleavable rocks. They develop byrecrystallization after rock flowage has ceased, but probablywhile the rock is still under high pressure and temperature, asis evidenced by their high specific gravity and characteristic 1 Adams, F. D., Report on the geology of a portion of the Laurentian area lyingto the north of the island of Montreal: Ann. Rept. Geol. Survey of Canada, Vol. 8,pt. J, 1896, p. 85 et seq. PORPHYRITIC TEXTURE 91 occurrence in the proximity of intrusive igneous rocks. Theirlate development by recrystallization is shown by the following-considerations: (1) They appear in rocks clearly derived by rockflowage from others originally lacking such minerals. (2) Theyfrequently lie at large angles to the prevailing cleavage in therock. (3) They do not show the degree of mechanical deforma-tion that they would necessarily have possessed had they devel-oped in their present positions before flowage had ceased. Many. Flu. 44. Photomicrograph of chloritoid crystal in micaceous and quartzose schistfrom Black Hills. The chloritoid crystal here shown has developed later thanthe rock flowage producing the prevailing cleavage of the rock. The chloritoidhas grown at the expense of the other constituents of the rock, using all thematerial necessary for its growth and leaving the excess of material in the formof inclusions, which retain their dimensional parallelism with the prevailingrock cleavage. of the crystals are long and acicular, and would surely have beenbroken if any considerable movement had occurred subsequentto their development. (4) They include, within their boundaries,minerals in part similar to those in the remainder of the rock,and which have an arrangement of their greater diameters in the 92 STRUCTURAL GEOLOGY plane of rock cleavage, showing that to some degree at least theywere formed during rock flowage. (5) The mica and the otherconstituents of cleavable rocks, wh
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