214 TEANSAOTIONS OF THE [ArR. 24 



In the white and gray varieties of the rock, a small quantity 

 of argillaceous material coats the sand grains. The presence 

 of this thin coating, particularly when it is hematite, gives to 

 the outlines of the grains a clearness that they would otherwise 

 lack. When, as is sometimes the case, the coating is absent it 

 becomes difficult to draw any line between the grain and the 

 surrounding cement. 



The rock affords an excellent example of the secondary 

 enlargement of quartz grains described by Sorby, * Irving f 

 and others. About every sand grain quartz has been deposi- 

 ted until further growth was checked by contact with the 

 quartz forming about adjacent grains. This secondary quartz 

 always shows perfect optical continuity with the original quartz 

 of the grain about which it is deposited. As a rule there is no 

 opportunity for the development of crystal outlines, the process 

 being too complete, but one or two faces are occasionally 

 formed. 



As a result of this process the rock is thoroughly indurated, 

 being practically a solid mass of quartz. 



PYROXENE ROCKS. 



Near the base of the ci-ystalline limestone, and seemingly 

 interstratified with it, is a body of schistose, highly contorted 

 rocks of somewhat doubtful character. In the field they were 

 taken for metamorphosed sediments, and no particular attention 

 was paid to them. But a microscopic study reveals a mineralogi- 

 cal composition that suggests the possibility of an igneous 

 origin. Against such a supposition, however, strong evidence 

 is afforded by the gradual transition often seen from limestone 

 to pyroxene rock. While there is considerable divei'sity in the 

 composition of different portions of these rocks, one variety 

 is particularly abundant. It consists of feldspar, quartz, 

 monoclinic pyroxene, mica, titanite, apatite, graphite, pyrrhotite, 

 and pyrite. 



Both orthoclase and plagioclase are present, sometimes one, 

 sometimes the other predominating. In some sections they are 

 very fresh, in others completely altered to muscovite and kaolin. 

 Quartz is usually in small quantity' or lacking. In a single 

 section, where it is uncommonly abundant, it shows a serpen- 

 tinous alteration along cracks, similar to that described and 



* Sorby, H. C, Proc. Gool. Soc Lond., 1880, p. 6-2. 



t Irvins, R. D., Am. Jour. Scionco. III., XXV., p. 40i. Bulletin 8, U. S. G. S., 

 5th Ann. liep-, U. S. G. S., p. 218. 



