FAEMINGTON AROH^AN BODY. 379 



structureless one. The orig'mal sheets of mica may still be traced through 

 the rock, but they are twisted and broken and crowded into such positions 

 that at first the rock looks like a very coarse granite. This same transition, 

 between schists and structureless rocks, the result "^f pressure, is also noted 

 in the East Humboldt Range, Nevada. Above the coarse mica-gneisses are 

 a series of gray gneisses, in which the feldspar and quartz are both white, 

 but which are f:irther interesting as containing freely-disseminated micro- 

 scopic garnets, which, under the microscope, are seen to be rich in minute 

 cracks, and to be more or less metamorphosed into chlorite; muscovite is the 

 mica of this member. A little higher in the series is another gneiss, still 

 containing a predominance of mica, but also a little hornblende; it is also 

 rich in garnets, which likewise show the transition into chlorite. Above 

 these are a heavy series of dark-green hornblendic gneisses, containing 

 little hornblende, but rich in feldspar and apatite; besides these minerals, 

 they also contain a considerable proportion of zircon, never, however, large 

 enough to be visible to the naked eye ; the mineral, however, is distinctly 

 the same that has been observed in other rocks, where its existence has 

 been proved chemically. 



The two general groups of which this Archaean body is formed, there- 

 fore, consist, first, of alternating quartzites and gneisses, in which mica- 

 gneiss decidedly predominates, and which are further noticeable as contain- 

 ing garnet ; and the second or uppermost zone is that of the hornblendic 

 gneisses, containing zircon. The hornblendes in these latter are usually 

 more or less fibrous, always dark green, and are arranged with the broader 

 surfaces of the prisms parallel. Quartz and feldspar hold a .very variable 

 position in this series; at the top, the rock is really an amphibole rock, con- 

 taining sparse grains of quartz, but no feldspar ; a little lower, quartz par- 

 takes largely in the composition, .but feldspar is quite rare. Lower still in 

 the region of the mica-gneisses, feldspar forms a full quarter of the rock, 

 but there is present with the hornblende considerable mica. Apatite seems 

 to be a characteristic of those rocks in which the mica does not exist. 

 Among the gneisses are zones varying from 50 to 100 feet in thickness. It 

 would seem, from the results of irregular pressure, both here and in the East 

 Humboldt Range, that the schistose structure can be easily broken up by 



