THE rEKIDUTITE. I35 



dikes intersect the schists, whereas all dikes, except the very freshest of the 

 diabases, are absent from the peridotite. These tacts would indicate that 

 the ])eridotite is an irruptivc of later ag-e than the schists. Dr. Wads^^•ol•tll 

 has recently described an irrnjjtive contact of tlie serpentine with the sciiists, 

 and a dike of the peridotite intruding diabase, diorite, and felsite. There 

 can l)c' no question, then, that the peridotite and its derivatives are voiuiger 

 than the Deer Lake Kitchi sciiists. From the relation of the various dike 

 masses to it, it would seem to be the latest intrusion in tliis area, with the 

 exception of the fresh diabases, whose irruption continued until the close 

 of Upj)er Jlarquette time. 



The freshest peridotite obtained in this area came from a large bluff near 

 the center of the E. h sec. 27, T. 48 N., R. 27 W. The rock is composed 

 of fairly well preserved diallage, olivine, magnetite, and plagioclase. The 

 olivine is in well-defined crystals, embedded in large plates of pale-pink, 

 almost colorless diallage that nearly fill the section. In the small inter- 

 spaces between the diallages occui-s the plagioclase, as a Meakly i-efractiug, 

 altered, white or colorless substance, occupying the same relation with respect 

 to the diallage and olivine as glass does to the crystals in a hypocrystalline 

 rock. The olivine and diallage are both serpentiuized in part, and the dial- 

 lage is in places uralitized and chloritized, especially near its contact with 

 the plagioclase. Tlie small amount of magnetite present is found amongst 

 these decomposition products, as are also a few flakes of biotite. Calcite 

 fills cracks in the other components and the spaces left between them. The 

 rock has the composition, but not the structure, of wehrlite. Its analysis, 

 made by W. F. Hillebrand in the Survey laboratory, is given on tlie next 

 page. 



From this analysis it is seen that the rock is more altered than would 

 be judged from the investigation of its single thin section at hand. It is also 

 noticeable that, like many other peridotites, it contains small percentages of 

 several rare metals, as titanium, chromium, manganese, and nickel, besides 

 traces of strontium and barium. 



From most of the sections of specimens taken from the Opin perido- 

 tite the olivine and diallage or other pyroxene have entirely disappeared 

 and serpentine or dolomite has taken their places. In many of these the 



