ARCHEAl^ IN FELOH MOUNTAIN DISTRICT. 395 



In its low silica and lime, and high iron and magnesia, this rock differs 

 in important particulars from the granites, to which in its mineral com- 

 position it is allied. In these respects, as well as in the great excess of 

 potash over soda, it closely approximates the composition of certain clay 

 slates.^ 



The original character of the mica-schists is indeterminate. They may 

 be altered sediments, as the chemical analysis indicates, but if so they no 

 longer contain any material which can be proved to be in its original form, 

 and in view of the complete recrystallization, for which the evidence is clear 

 and striking, this could not be expected. Their mineralogical relationship 

 and close association with the granites and gneisses is perhaps a reason for 

 regarding them as autoclastic rocks, derived from originally massive granites 

 by dynamic metamorphism. If this be true, then the crust movements 

 which crushed the parent granite belong to pre-Algonkian time, for the later 

 stresses which folded and brought the schists into faulted contact with the 

 Randville and Sturgeon formations found them with a parallel foliation 

 which it bent and crumpled, and no period of great stress earlier than this 

 is known in Algonkian time. The complete recrystallization may be 

 referred with probability to the period of quiescence following the faulting 

 and folding, during which also occurred the recomposition of the older 

 Algonkian formations. 



(4) The amphibolites or hornblende-gneisses are widely and abundantly 

 represented in the Archean. Macroscopically they are black or dark- 

 green rocks of medium to faiidy coarse grain, the fresh fractures of which 

 glisten with the cleavage surfaces of hornblende, which is much the most 

 abundant and often the only recognizable constituent. They are universally 

 foliated parallel to the foliation of the associated gneisses, and exhibit, 

 but in a more marked degree, the same varieties of structure. The folia- 

 tion is easily recognized by the eye as due to the parallel arrangement 

 of the hornblende prisms. Depending mainly upon the position of the 

 hornblendes relative to the other constituents, the structure is either 

 of the plane-parallel or linear-parallel type, the latter often superbly 

 developed. 



The essential constituents of these rocks are common green horn- 

 blende, plagioclase, biotite, and quartz. The structure is thoroughly crys- 



' See analyses quoted by Kemp, Handbook of Rocks, p. 107, nos. 4 and 5. 



