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Cincinnati Society of Natural History. 



can be determined, another term is prefixed to denote the 

 rock type from which they have been derived. In this 

 chapter it will be shown that the Port Deposit rock is prob- 

 ably of igneous origin, and that it has had a secondary foliated 

 structure produced through dynamic metamorphism. Since 

 it furthermore has both the chemical and mineralogical com- 

 position of a normal granite it is considered most suitable to 

 designate it as a granite-gneiss. 



The original character of this granite-gneiss was once very 

 similar to that of the Rowlandville granite type already 

 described, and it still contains the same essential minerals, 

 such as feldspar, quartz and biotite. The more intense action 

 of pressure has, however, here been to crush the constituents, 

 and to produce cataclastic structures rather than to bring 

 about mineralogical changes, such as epidotization. 



The feldspar belongs to both the alkaline and to the lime 

 soda series. There seems to be in the southern area a greater 

 development of potash feldspar in the form of microcline 

 than in the Rowlandville rocks. This mineral show r s the 

 characteristic cross hatching in basal sections, and it appears 

 striated in other sections. Though it often occurs in masses 

 of considerable size, it is more usually broken into smaller 

 fragments. The plagioclase feldspar shows fine twinning 

 lamellae, and usually the sections have low extinction angles, 

 which would suggest that it belonged to the albite oligoclase 

 series. There is also present a considerable amount of 

 untwinned allotriomorphic feldspar, usually much altered to 

 muscovite and epidote, which may be orthoclase or untwinned 

 albite. In some sections of this latter feldspar, there occur 

 small more or less rounded areas of microcline. Undoubted 

 examples of microperthite are comparatively rare. In some 

 cases the feldspars form micro-pegmatitic intergrowths with 

 quartz, where the grains of the latter mineral occasionally 

 have a fan-like arrangement. 



All these feldspars exhibit pressure effects. They show 

 modulatory extinction and peripheral granulation. In some, 

 the crushing, combined with chemical change, have caused a 

 very considerable development of the albite mosaic, composed 

 of very fine interlocking grains, which look not unlike quartz. 

 The alteration products of this mineral are epidote and 



