GEOLOGIC HISTOBY 323 



The pre- Cambrian arkose, by crystallization and metasomation, was 

 converted into a gneiss. Induration and crystallization, a mass move- 

 ment and a molecular movement in response to pressure, have converted 

 the Cambrian beach deposit into a conglomerate schist and a quartzite. 

 Where the sand was originally impure metasomation has been an acces- 

 sory process, and the products, a sericitic quartz-schist. 



Crystallization and an interchange through the agency of ocean water 

 of calcium and magnesium carbonates have been the processes chiefly 

 active in converting the calcareous precipitates and sediments into 

 marble or crystalline limestone. 



The impurities present have crystallized into silicates, such as phlogo- 

 pite (chiefly), tremolite, and diopside, and all evidence of organisms 

 which may in some degree have assisted in the original formation of the 

 rock is lost. 



Crystallization and metasomation have been the chief processes con- 

 cerned in the production of the Wissahickon mica-schist and mica-gneiss 

 from the arcosic and argillaceous material. Muscovite and quartz have 

 developed from the feldspathic material present and a schist produced, 

 or the feldspathic material has recrystallized as feldspar and the ferro- 

 magnesian material present in the sediment as biotite, and the resulting 

 rock type is a mica-gneiss. Both injection and impregnation have locally 

 taken part in the development of the Wissahickon mica-gneiss, produc- 

 ing a banding which very perfectly simulates gneissic banding. The 

 fact that the new minerals which have crystallized from the impurities 

 of the sediments are exclusively minerals, which, by the parallel orien- 

 tation of their longer axes normal to the direction of compression dimin- 

 ish the horizontal dimension of the formation, shows that their develop- 

 ment like the folding was in response to pressure and was a means of 

 relieving strain. The intensity of the pressure is further indicated by 

 the fact that muscovite and biotite are developed in such excess to the 

 almost complete exclusion of hornblende and chlorite. The develop- 

 ment of new minerals has thus been accompanied by the development 

 of new structures, namely, schistosity and gneissic banding. 



The flattening and rotation of the original minerals of the sediments, 

 such as quartz or feldspar, as well as parallel orientation, described above, 

 of the new minerals, takes part in the production of these structures. 

 This deformation, which results in schistosity, is microscopic as well as 

 megascopic. A strained molecular condition, giving rise to undulatory 

 extinction, peripheral granulation, complete granulation, and rotation 

 of the grains are stages in the process. 



Granulation is not a feature of the Piedmont sedimentary formations. 



