Structural geology . n in thesecondary rock. A shale or mud may have no mica; a phyllite,its altered equivalent, may have as high as 50%, by weight, ofmica. Chemical analysis shows that this change may occur in someinstances with little addition or subtraction of materials. A correctinference is that the new minerals of the hornblende and micatypes have developed principally from the recrystallization ofsubstances already in the rock mass. Even where there is quan-titative evidence that substances have been introduced or ex-tracted, the mass has still been recrystallized. Since hornblende 80 S
Structural geology . n in thesecondary rock. A shale or mud may have no mica; a phyllite,its altered equivalent, may have as high as 50%, by weight, ofmica. Chemical analysis shows that this change may occur in someinstances with little addition or subtraction of materials. A correctinference is that the new minerals of the hornblende and micatypes have developed principally from the recrystallization ofsubstances already in the rock mass. Even where there is quan-titative evidence that substances have been introduced or ex-tracted, the mass has still been recrystallized. Since hornblende 80 STRUCTURAL GEOLOGY and mica are the common minerals producing the best rock cleav-age, it must be concluded that recrystallization is the importantprocess in the development of parallelism of the mineral constit-uents. Corroborative evidence of the importance of recrystallization isthe general lack of fractures or other strain effects in the mineralsof a cleavable rock, such as would be expected if the parallelism had. Fig. 38. Photomicrograph of micaceous and quartzose schist showing recrystal-lized quartz. From Hoosac, Mass. The view illustrates in detail the relationof recrystallized quartz grains to recrystallized mica flakes. The mica flakes forthe most part separate different quartz individuals, hut they may be seen tobound two or more individuals and to project well into them. It is not prob-able that such a relation could be brought about by granulation, slicing, orgliding, and it seems best explained by recrystallization. been brought about entirely or largely by mechanical may be inferred, then, that some constructive process, whichmay be called generally recrystallization, has been at work. Most of the mineral particles in the cleavable rocks are in-dividually larger than the particles in the same rocks beforeflowage had occurred. For instance, the gradation of a shale to aphyllite means an increase in the size of the grains. Recrystalliza-tion is the con
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishernewyo, bookyear1913
