486 THE MARQUETTE IROX-BEARING DISTRICT. 



garnet, and quartz. The garnets and amphibole are nnquestionablv new 

 products. Both are idiomorphic, and lioth occur in large crj^stals. The 

 biotite is a red-brown variety that apparently resulted from the decom- 

 position of portions of the original materials of the rock, especially where 

 these contained some proportion of sedimentary material. Where these 

 new products are present in large quantity the original structure of the 

 tutfs has nearly, if not quite, disappeared. Where they are scarce the rock 

 retains its tuffaceous character. In this latter case fragments of plagioclase 

 and crystals of this mineral are found to be embedded in an aggregate of 

 small fragments of the same substance, flakes of biotite, spicules and plates 

 of chlorite, fibers of green hornblende, grains of magnetite, and an inter- 

 stitial aggregate of cryptocrystalliue quartz. Where the tuffaceous material 

 is mixed with sedimentarv substances there are found also, in the aggregate, 

 rounded grains of quartz. In the foliated phases of the tuffs and mixed 

 tuffs and sediments the biotite exists in very large quantity, so large, indeed, 

 that these phases often resemble biotite-schists. In them a few quartz 

 grains are observable, and a fine-grained matrix that appears to have been 

 derived fi'om a fine-grained, clastic, interstitial filling between the larger 

 grains of the original rock. 



