208 THE MAKQUETTl': IKONBEAKING DISTRICT. 



MICACEOUS AMPHIBOLE-SCIIISTS. 



There is a third class of schists that possess at the same time some of 

 the characteristics of the hornblendic schists and others of the micaceous 

 schists. Macroscopically theA' resemble the latter. They are finely and 

 evenly banded arenaceous rocks of a light-gray color. Under the microscope 

 they appear iBore like the hornblendic schists. Quartz in larg-e quantity, 

 altered plagioclase, brown biotite, and green amphibole are all present in 

 them. Tlie last three components vary in amount, but all are in large 

 (piantity. The plagioclase and quartz are in irregular and often jagged 

 grains, elong-ated in one direction a little more than in others. The biotite, 

 liowever, and usually the hornblende, always occurs in long, narrow flakes 

 between the other components, and it is due to the fact that the longer 

 directions of these flakes are always parallel that the rocks are foliated. 

 Tlie bands differ from each other only in the amount of hornblende and 

 biotite in them. The lighter bands ai-e devoid of these minerals, while the 

 dark ones contain them in great abundance. In one or two instances, 

 where the banding of the schists is very obscm-e, the structure is granitic 

 in so far as the quartz and plagioclase are concerned. These two minerals 

 occur in irregular grains that are separated from one another by numerous 

 flakes of biotite and honablende. The latter minerals lie with their longer 

 axes approximately parallel to the bounding planes of tlie quartz and feld- 

 spar grains, as if they had been forced into this position by pressure acting 

 perpendicularh^ to their predominating direction. The feldspars are more 

 or less altered to a mosaic of quartz and sericite or kaolin, or of quartz and 

 clear plagioclase. 



ORKUN. 



All the hornblendic schists appear to have been produced by the mashing 

 of some original basic crystalline. It is not possible to ascertain jjositively 

 that cpiartz was not a constituent of the parent rock, but from the fact that 

 it is so often a i)roduct of the decomposition of the original plagioclase it 

 is thouiilit ])rob;ible that much of it, in both the micaceous and the non- 

 micaceous hornblendic schists, is a secondary product. The biotite and 

 hornblende are also secondary, but the mineral from which they were 

 derived is not known. It may very likely have been augite. 



