THE IGNEOUS HOCKS. 491 



mediuin-g-rained, and rarely fino-g-raiued. In many of them the diabasic 

 structure is still very plainly visible, while in others it has been lost throu<>-h 

 alteration and throug-h mashing. The more massive of the gr(ienstoTies, 

 more particularly those occurring in the knobs, are the miners' "diorites." 



MaoroscojHcally the material of the eastern knobs is a. liglit or dark 

 grayish-green, mediumly coarse grained rock, that is rarely massive. Usu- 

 ally some trace of foliation may be distinguished in the hand specimens. 

 Frequently the schistosity is so sliglit in amount that it is recognizable 

 only in the ledge. In other cases the rocks are highly schistose, Avhen they 

 merit the name of greenstone-schists. While the schistosity of many of the 

 knobs is more iwonouuced around their peripheries and along joint planes 

 than elsewhere within their masses, the schistose greenstone mav occur 

 anywhere within a greenstone area, even in the midst of great areas of 

 perfectly massive rock. The intimate relations existing between the schis- 

 tose and massive greenstones indicate conclusively that both are phases of 

 the same rock mass, which yielded here and there to some force, with the 

 result that motion was set up between its parts, which have, as a conse- 

 quence, become schistose. Along joint planes are often shear zones, and at 

 these places the rocks are as completely schistose as are chlorite-schists. 



In nearly all specimens of the eastern knobs, even in some of those 

 that are schistose, the diabasic structure may be detected. When not 

 apparent in the hand specimen, it can nearly always be observed in the 

 thin section, although in most cases all traces of the original components 

 of the rocks have disappeared and their places have been taken by 

 secondary substances. In the freshest of the rocks, which usually come 

 from the centers of the knobs, cores of pale-^^olet or almost colorless 

 augite, surrounded by rims of green hornblende, altered plagioclase, chlo- 

 rite, epidote, and often calcite, are arranged in the typical ophitic manner. 

 The plagioclase, by its alteration, has given rise to the chlorite, epidote, 

 and calcite, and often to a sericitic substance, which in some instances 

 appears to be genuine muscovite. 



A typical knob, from the petrographical standpoint, and one which 

 exhibits all the varieties met with in the eastern knobs, is that which extends 

 from Gunpowder Lake, at the east quarter post of sec. 11, T. 47 N., R. 27 W. 



