THE IGNEOUS ROCKS. 503 



mashing but gTeat chemical changes as well, for they are now often pure 

 chlorite-schists, composed of a solid mesh of small chlorite fibers, between 

 which are occasionally small areas of quartz mosaic, tiny grains of magnet- 

 ite, biotite flakes, and pyrite crystals. In some cases quartz mosaics pseudo- 

 morph the oi'iginal plagioclase grains, and in others large garnets occur 

 scattered indiscriminately through the rocks. These latter are well seen at 

 the Michigamme and Spun- mines and on the borders of some of the boss- 

 like greenstone masses in Humboldt Mountain, where they have been 

 regai-ded as possibly the result of contact action, since they are often as 

 well developed in the sedimentary beds contiguous to the greenstones as they 

 are in the greenstones themselves The garnets in the greenstpnes exhibit 

 no anomalies, so far as seen. They are almost colorless, isotropic bodies, 

 crossed by irregular cracks and containing as inclusions a great many 

 irregular grains of magnetite and very irregularly outlined colorless masses 

 that under the crossed nicols appear to be quartz and plagioclase. At the 

 Spurr mine many of the garnets are more or less completely changed to 

 chlorite, as described by Pumpelly in 1875. 



The very schistose phases of the western greenstones, where they are 

 not on the contact with the sedimentary rocks, are almost typical horn- 

 blende-schists. This is especially true of those greenstones in the Republic 

 trough. As we pass southward from the Magnetic mine, in sec. 20, T. 47 N., 

 R. 30 W. (Atlas Sheet VII), it is noticed that the greenstones become more 

 and more schistose and at the same time more crystalline. Their quartz- 

 ose component increases in quantity until in some of the rocks, especially 

 those in the vicinity of Republic (Atlas Sheet XI), it makes up a large 

 proportion of the rock masses. Many of these rocks are composed of 

 crystals of bright-green amphibole, often with idiomorphic cross-sections, 

 large lenticular grains of quartz and plates of epidote, and, between 

 these, masses of altered plagioclase, consisting largely of kaolin, epidote 

 or saussurite, and biotite. Leucoxene and magnetite also occur in the 

 schists, the former mineral with the habit of spliene and the latter with 

 very irregular outlines. Others of the schists contain large quantities of a 

 bright-green hornblende with extremely ragged contours and groups of this 

 mineral composed of numbers of small grains and spicules of amphibole 



