PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 271 



Diabase. Basalt.] 



Meg. A compact, hard, brownish red, siliceous, aphanitic rock, which holds 

 porphyritic crystals of quartz and red feldspar. 



Mic. The porphyritic feldspars are much decayed and reddened. The porphy- 

 ritic quartzes are considerably rounded and are sometimes penetrated by little 

 embayments from the groundmass. The groundmass is similar to that of most of 

 the aporhyolytes already described, i. c., a little magnetite, hematite, much minute 

 cloudy feldspathic material, the whole section reddened, and all in a background of 

 quartz, which is usually in poikilitic areas holding the other materials of the ground- 

 mass. The section shows some irregular areas of finely crystalline quartz from 

 which most of the other materials of the groundmass are lacking. In ordinary light 

 these areas are transparent and colorless. 



One section. 



Age. Cabotian; red-rock series. u. s. G. 



No. 231. DIABASE. 



From a dike of basaltic doleryte, cutting rock No. 230, a short distance east of the mouth of Red Rock 

 creek, embracing patches of the red rock (No. 230). Compare No. 1818. 

 Ref. Annual Report, ix, page 58. 



Very fine-grained, nearly black, with isolated nodules of pyrite. 



Mic. The slide is too thick for such examination. The feldspar is in the form 

 of lath-shaped crystals without distinct terminations. Much magnetite is visible as 

 small roundish grains. 



One section. 



Age. Cabotian; probably an apophysis from the great gabbro mass. N. H. w. 



No. 232. BASALT. (Amygdaloidal.) 



Is the first rock that appears in the pebbly beach east of the rock of the Red Rock point, probably in 

 S. W. y sec. 25, T. G3-5 E. Compare No. 1823. 



Ref. Annual Report, ix, pages 58, 59; Annual Report, x, page 47. 



A reddish brown, fine-grained, compact or amygdaloidal rock, the 

 amygdules being white, coated with green or entirely green. Apparently underlies 

 No. 230, and resembles some of the compact brown rock seen at Duluth and at many 

 intermediate points. 



Mic. The feldspars and all the other minerals are very much stained with ferric 

 oxide, and penetrated by other impurities. The section between crossed nicols is 

 nearly opaque. 



There are remnants of olivine, and of aiigite, and a considerable quantity of 

 magnetite, the last prevailing in the vicinity of the olivine, and enwrapping it. In 

 general the feldspars are quite perfect in their outlines, the reddened substance 

 which embraces them being probably the uncrystallized remnant of the magma. 

 The old olivine grains, entirely altered, sometimes show nearly regular crystalline 



