MUSEUM OF COMPARATIVE ZOOLOGY. 



47 



At Republic Mountain, on the southwest side, a garnetiferous "dioritc " 

 was seen in direct contact with the jasper, whicli was much twisted and 

 contorted. The ore associated with the jasper was magnetic. The 

 " diorito " was found to be intrusive here and elsewhere about HepubHc 

 Mountain, breaking through and uphfting the overlying rock, whoso 

 hiniina3 it is seen to cut obliquely. It also sends stringers and dikes 

 into the schist and jasper. As the " diorite" passed approximately alon^>- 

 the lamination of the schists, and is foliated parallel to its walls, it is easy 

 to see how those who believed that, if a rock was '*stri2:)ed," it was ^:>nVMa 

 facie evidence that it was a sedimentary one, should overlook the eruptive 

 characters. Most of the " diorites " here contain garnets, this mineral 

 being found principally along the edges of the intrusion, while the cen- 

 tre was nearly, if not entirely, free from it. The schist, in like manner, 

 near the *' diorite," also frequently contains garnets, both rocks aj^pcariug 

 to have mutually reacted upon each other. Under the microscope, one 

 of these ** diorites " (124) is seen to be composed of actinolite, garnet, 

 quartz, and biotite. The arrangement, forUi, and relations of these indi- 

 cate that none of them are original constituents of the rock, but all are 

 the products of altei^ation. 



A dike of dark micaceous "diorito" (123), some ten inches in width, 

 was seen near this place, which contained garnet crystals varying from 

 one half an inch to two inches in diameter, averagiLg nbout three 

 fourths of an inch. These garnets, like the others observed, are dode- 

 cahedrons. Microscopically it is seen to be composed of biotite, quartz, 

 feldspar, and magnetite (?) with the enclosed garnet crystals. The 

 black grains have the microscopic charactei-s of magnetite, although tlio 

 powder is not magnetic. The biotite is the predominating mineral, and 

 all tlic present constituents, except, perhaps, the supposed magnetite, 

 appear to be secondary products. The garnet is filled with the magnet- 

 ite (]). These black inclusions, so far as we are aware, have been taken 

 for magnetite,* except some observed by Professor Pnmpelly.f In po- 

 larized light the garnet is seen to contain abundantly the same grains ot 

 quartz and feldspar that the groundmass does ; it also holds some biotite. 

 Tiie "magnetic siliceous schist" of Mr. Brooks (12G)niear the granite 

 (128, 129, 130) southwest of Republic Mountain (p. 53), is seen micro- 

 scopically to be composed of actinolite, hornblende, magnetite, and 

 garnet. The two first form the major portion ,of the section. The 

 garnet is filled with needles of actinolite. Southeast of Republic Moun- 

 tain, the same rock (No. 100, p. 53) contain-s longer and better formed 



* Geology of AAlsc, III. COS. 



t Am. Jour. Sci., (3,) X. 20. 



