

58 



BULLETIN OF THE 



part. The difference now between the two rocks is perhaps due to the 

 action of the granite upon the schist. Microscopically this schist (353) 

 is composed of biotite, quartz, magnetite, and a niiiscovitedike mineral. 

 The quartz contains inclusions of the other minerals, and fluid cavities 

 with bubbles in exceedingly active motion compared with the usual rate 

 of movement. The fluid cavities are not numerous. The part filled 

 with quartz and the muscovite-like mineral appears to be a portion of 

 the original fine sediment (mud) that held the coarser material. The 

 alteration and crystallization of this argillaceous detritus give with the 

 original quartz grains the present texture, approaching closely a gneiss. 



East of the same mine the gneiss (344) was found dipping N. GO'' 

 W. 67°, and is finely foliated. This gneiss is cut through by intrusive 

 granite, and by dikes of diabase and ''diorite" (343, 345, 34G). The 

 latter cut the granite as well as the gneiss. (Sec page 45.) The gneiss 

 is composed of biotite, quartz, and the same muscovitedike mineral as tiie 

 schist. No. 353, which in fact it closely resembles. The quartz con- 

 tains microlites, scales, and grains of the other minerals and fluid cavi- 

 ties. The same decomposed material cementing the quartz grains is 

 found in this as in the schist. So far as we can tell by microscopic ob- 

 servation, we regard this rock as sedimentary, and perhaps only a more 

 highly nu^tamorphosed condition of tlie adjacent schist. As its relations 

 to any rock older than itself were not observed, of course no definite 

 statement can be made. The supposed sedimentary material forming 

 the cement and the base out of which the mica, in part at least, has 

 crystallized, can arise from the decomposition of feldspar crystals in 

 their original position, and a material nndistinguishable from it is seen 

 frequently to have been formed thus in eruptive rocks. We base our 

 conclusion that the rock is sedimentary upon the general structure of 

 the rock, especially upon the form and relations of the quartz grains to 



the rest of the constituents. 



The term quartzite is, we believe, when employed in its proper use, 

 restricted t'> an indurated sandstone, and in this sense we employ it. 

 We believe this to be more in accordance with the generally accepted 

 use of the term, although, in practice, Messrs. Zirkel, Lasaulx, Hawes 

 (a pupil of Lasaulx), and other lithologists, employ it also to designate 

 quartz veinstones and other forms of chemically deposited quartz. ' Dr. 

 Wichmann, it would seem, employed it only for rocks which have no in- 

 terstitial or cementing substance remaining imcrystallized between the 

 quartz grains, lie classes them under the head of non-fragmental rocks, 

 saying that his other class comprises all those " which have been formed 



