164 TUE MAKQUETTE lEON-BEAElNG DISTEICT. 



to tlic foliation the rock ajjpears to be a typical sericite-schist or talcose 

 schist. 



Except for the arraug-ement of the pebble-like masses in bands, there 

 is little in the inacrosc()i)ic api)earance of these rocks that resembles the 

 bed(lin<>- of water-deposited materials. In two or three hand specimens a 

 fine banding- was detected, a slight difference in tint between the alternate 

 layers leading to their recog-nition, but this is all. 



Microscopical. — Tlie couglomeratic green schists are so much decom})osed 

 that it is difficult to learn from their thin sections much more concerning 

 their original character than can be learned from their study in the field. 



The i)ebble-like masses scattered through the schists are fragments of 

 a purplish-pink, fine-grained rock, speckled with tiny green dots of chlorite 

 and red or white ones of altered plagioclase. Enough of the feldspar 

 remains to exhibit traces of twinning bars, although most of it has been 

 rej^laced bv sericite, calcite, and quartz. These plagioclases Avere origi- 

 nally well-outlined crystals. They lie in a groundmass i-omposed of small 

 feldspar laths, grains of epidote, and a weakly polarizing felsitic substance 

 that is probably a devitrified glass. The rock of which the pebbles were 

 a part was probably a porj)hyrite, unlike anything that has yet been found 

 in place within the limits of the district. 



The schistose groundmass in which the })ebbles are embedded consists 

 of sharplv angular fragments and complete crystals of altered plagioclase 

 in a matrix composed of much chlorite and sericite, small fragments and 

 crystals of j^lagioclase, always some calcite, and a fine-grained mosaic of 

 isecondary quartz. To these is sometimes added epidote in grains and 

 plates. It is noteworthy that in these I'ocks chlorite has replaced the 

 original iron-bearing silicate, while in the tuffs of the Mona schists to 

 the east these silicates are now represented by luirnblende. Whether this 

 diffei-ence in composition is due to differences in the nature of the alteration 

 processes to which the different rocks have been subjected, or to the fact 

 that the rocks of the greenstone-conglomerate area have suffered the effects 

 of weathering to a greater extent than the Mona schists, is not certain. It 

 seems most ])robable, however, that both causes are responsible for the 

 differences. 



