162 THE VERMILION IKON-BEARING DISTRICT. 



the sotith flank of the Twin Peaks ridge. Here the texture of the 

 greenstone is characteristically ophitic, a secondary hornblende taking 

 the place of the original pyroxene. When metamorphosed by the gabbro 

 we find the ophitic texture perfectly preserved with, however, a large 

 quantity of biotite as a secondary product. This biotite has accumulated 

 around the edges of the hornblende between the hornblende and the feld- 

 spar, and is especially concentrated along evident shearing planes where 

 normally — that is, in the greenstone unaffected by the gabbro — one would 

 find a large amount of chlorite derived from the hornblende. With rocks 

 like the above there is associated another, showing the ophitic texture 

 poorly preserved and with brownish-green, massive hornblende constituting 

 most of the rock, and with hypersthene occurring in more or less porphyritic 

 areas. This hypersthene is very fresh and seems to be a product of the 

 action of the gabbro on the greenstone. In other cases ophitic textured 

 greenstones seem to contain a very much larger amount of a brownish- 

 green hornblende and magnetite than these greenstones normally contain, 

 and in this instance the large quantity of magnetite, and possibly also the 

 brown hornblende, is assumed to be due to the action of the gabbro. In 

 general, there are produced from the greenstone, by metamorphism of the 

 gabbro, rocks which contain a large percentage of biotite and varying 

 quantities of hypersthene and magnetite. As a result of their mineralogic 

 character such rocks have a rusty-brown color, and the texture, although 

 distinctly ophitic, is inclined to become granular as the new minerals 

 increase in quantity. These rocks disintegrate much more readily than do 

 the greenstones. 



RELATION OF ELY GREElSrSTOlSTB TO ADJACENT FOEMATIOlSrS. 



The relations of the greenstone complex to the adjacent formations 

 have already been briefly stated, but will be recapitulated. Wherever the 

 greenstone complex is in contact with any sediments all the larger masses 

 lie above and are infolded in it. When these sedimentaries are normal 

 clastic deposits they lie above and contain iiumerous fragments of the green- 

 stones, showing that the greenstone complex is the older formation. When 

 the greenstones lie next to other igneous rocks they are found to be 

 penetrated by them. Hence all of the relations of the greenstone complex 

 to the -various adjacent formations prove its greater age. A detailed 



