THE IGNIDOUS HOCKS. 513 



wliicli may often be distinguished from the true soapstones by the coarse 

 grit in their })owder, due to the presence of the fragmental C[uartz grains. 



A small numlKir of the dark-colored dike greenstones of the district are 

 abnormal in that they contain no aniphibole. Their original nature has not 

 been made out. As at present constituted they are composed of altered 

 feldsjiar grains in a mosaic matrix of quartz and plagioclase, containing 

 reddish-brown biotite of the usual kind. Calcite, sericite, kaolin, chlorite, 

 and epidote are mingled with the other components in large quantities, so 

 that the rocks as they now exist consist exclusively of secondary pi'oducts. 

 These rocks remind one strongly of the less highly foliated varieties of the 

 micaceous schists iu the Basement Complex. 



CONTACT EFFECTS. 



In the iield work on the dike rocks extensive and careful search was 

 made for contact effects in the intruded rocks, but the only evidences that 

 were thought possibly to indicate contact action were noted on Mount 

 Humboldt, upon Republic Mountain, and near the Magnetic mine. The 

 Mount Humboldt occurrence is the most characteristic. Here we find 

 cutting the grtiueritic schists of the northwest portion of the hill a highly 

 schistose, dark-green chlorite-schist, which on its edges is thickly besprinkled 

 with dodecahedral garnet crystals measuring about 2 mm. in diameter. 

 The griinerite-schists on the other side of the contact are likewise crowded 

 with similar crystals. Other crystals of the same kind, however, are often 

 met with in the schists at long distances from visible contacts with dikes, 

 but never so abundantly. Therefore the unusual concentrations of garnets 

 along the dikes are believed to be due to chemical interactions between the 

 two rocks subsequent to the intrusions. 



The chlorite-schist in the middle of the dikes is not essentially different 

 from the chlorite-schists already described. Near the contacts some of 

 the schists contain groups of griinerite needles identical with those in the 

 griineritic schists. Idiomorphic hornblende crystals are also occasionally 

 observed in them, and magnetite is everywhere present. Fibrous green 

 hornblende is a prominent component, and biotite is abundant, intermingled 

 with the amphibole in some specimens and in others accumulated around 



MON XXVIII 33 



