256 THE GEOLOGY OP MINNESOTA. 



[Diabase. Aporhyolyte . 



No. 202. DIABASE (with olivine). 



" Green, coarse doleryte, round the east point of Grand Marais; a low exposure in the coast line; with con- 

 cretions or inclusions of a finer grain. This terminates rather abruptly on the east, somewhat like a dike, when 

 in contact with No. 203; but it is not basaltic, nor is the contact abrupt. Nos. 202 and 203 change colors grad- 

 ually, and in fragments are mixed through a breccia of three or four feet wide." 



Ref. Annual Report, ix, pages 52, 53. 



Mcy. A black, heavy, rather coarse diabase. There are a few large porphyritic 

 feldspars, one of which is three-fourths of an inch across. The hand specimen also 

 contains a few small areas, aphanitic, and of a dark-brown color. These areas are 

 quite soft. They may be included fragments of a foreign rock or pseudamygdaloidal 

 areas of some soft earthy mineral. 



Mic. The section, which is a poor one, shows an ordinary diabase, considerably 

 altered. There are some dirty yellowish areas which represent original olivines. 



One section. 



Age. Cabotian. u. s. G. 



No. 203. APORHYOLYTE. 



Outcropping in the beach east of Grand Marais. It is the rock that gives red color to the beach for many 

 miles, by reason of the rapid formation of pebbles from the frequent cracks that intersect it. The pebbles also 

 are very durable. They are also abundant in the drift clays of the region. The rock dips 5 to 15 toward the 

 lake, and consequently in some instances, as viewed from the lake, appears horizontal by reason of the coincid- 

 ence of the coast line. 



Ref. Annual Report, ix, pages 53, 54, 55, 57; Annual Report, xiii, pages 100 (No. 162), 103; Bulletin viii, 

 page xxxiii. 



Meg. Very much like the Palisade rock (Nos. 138 and 139), and sometimes much 

 resembles the siliceous, slaty, gray rhyolyte of Beaver bay (Nos. 127, 528). 



Mic. The specimens bearing this number are rather more crystalline than the 

 Palisade rock. The thin sections show translucent, sharp, rod-like areas, recalling 

 an original feldspathic crystallization, and between them the angular spaces are 

 reddish and sub-opaque; but the whole rock is permeated with silica, which embraces 

 the rest of the undefined mineral contents in a coarse poikilitic nexus which darkens 

 simultaneously over large areas. 

 Two sections. 



Remark. The following analysis has been made of rock No. 203: 



SiO 2 73.58 



A1 2 O 3 13.36 



Fe 2 O, 3.78 



FeO .69 



CaO .81 



MgO .18 



K 2 2.48 



Na 2 O 2.42 



H 2 1.14 



Total 98.44 



Age. Cabotian; red-rock series. N . H . w. 



