IGNEOUS EOCKS OF STUEGEON EIVEE TONGUE. 483 



They difiPer from the latter in containing, as a rule, less quartz and a very 

 rauch greater abundance of epidote. The epidote is all secondary, as is also 

 the quartz, so that the only noticeable difference between the two sets of 

 greenstones is dependent upon differences in the nature of their alteration, 

 which in turn are probably the results of differences in environment. Both 

 sets of greenstones have been squeezed, but those in the Basement Complex 

 are associated with crystalline schists, while those in the Algonkian series 

 are associated with fragmental beds. 



In addition to hornblende, plagioclase, epidote, and a little quartz, 

 almost all the later greenstones contain biotite, small crj'stals of magnetite, 

 and irregular grains of ilmenite or of a titaniferous magnetite. Their 

 structure is schistose through the arrangement of the larger liornblendes 

 and biotites and the elongation of the feldspar grains in approximately 

 parallel directions. As a rule, their thin sections present no unusual features. 

 They all show dirty green hornblende plates, greenish-brown biotite flakes, 

 magnetite crystals, etc., embedded in a mass of irregular grains of decom- 

 posed plagioclase, the principal decomposition product of the feldspar being 

 in almost all cases epidote. 



Often the proportion of epidote present is A^ery great. It oeciu-s as 

 colorless crystals and grains scattered through the hornblende, and as light- 

 yellow plates and grains embedded in the mass of altered plagioclase. In 

 the rock at 500 paces east, 125 north, of the southwest corner of sec. 8, T. 42 

 N., R. 28 W. (PL LII), the replacement of the plagioclase by epidote has pro- 

 ceeded so far that no trace of the feldspar can be discovered. In the hand 

 specimen the rock is seen to be a massive mixture of black glistening horn- 

 blende crystals in a yellowish-green groundmass possessing a sugary texture. 

 In the thin section the hornblende is present as bluish-green plates that are 

 often idiomorphic in cross section. The groundmass in which they lie is 

 composed of epidote and quartz. The epidote is in large yellowish-green 

 irregularly-outlined plates, including particles of magnetite and small 

 rounded quartz grains. Most of the quartz is in isolated grains between the 

 epidote plates and in little nests of interlocking grains. Small magnetite 

 granules are scattered everywhere throughout the section, through all of the 

 components indiscriminately 



The coarser greenstones show plainly in the hand specimen the ophitic 

 structure, even where the rocks are schistose. In the section this structure 



