188 THE GEOLOGY OP MINNESOTA. 



[Diabase. Tuff. 



spar areas show the cross grating twinning of n/ici-orfinc. M<i</nt'fite in small grains 

 is present, and much dust-like magnetite and lieinaiite. It is not certain what com- 

 poses the dark mineral in the hand specimen. The slide shows a few small areas, 

 outlined by magnetite grains, which areas probably represent the dark mineral of 

 the hand specimen. In some cases the feldspar of the groundmass penetrates into 

 these areas, and again they are composed of very fine grains and fibres. 

 One section. 



Chemical analysis. The following analysis of this rock was made by Prof. J. A. Dodge, and first pub- 

 lished in the Thirteenth Annual Report, page 100 (No. 155). 



Si0 2 71.1.-) 



A1 2 O, 12.40 



Fe 2 O 3 5.21 



FeO .75 



CaO 1.90 



MgO 1.13 



K 2 O 2.40 



Na 2 O 1.70 



H 2 - 2.12 



Total 98.76 



The analysis (No. 1) quoted in Bulletin viii is not of this rock, but of No. 526, which is regarded as similar 

 to No. 124. 



Aye. Cabotian; granitic red rock. u. s. G. 



No. 124A. DIABASE. 



Dike rock, within No. 124. 



Kef. Annual Report, ix, page 32. 



Meg. A fine-grained, dark-gray, diabasic rock, with a somewhat earthy decayed 

 appearance. It resembles somewhat No. 122. 

 No section. 



Age. Manitou (?) r. s. G. 



No. 125. TUFF. (Altered.) 



Northwest from Beaver bay, S. E. % sec. 2, T. 55-8. 

 Kef. Annual Report, ix, page 32. 



Soft, reddish amygdaloid; explored for copper. Several test-holes and 

 surface trenches have been dug on various sides of a conical hill, made up of alter- 

 nating layers of reddish-brown firm rock (trap ?) and soft amygdaloid very much 

 like the layers that form the hill west of Agate bay. This amygdaloid is so soft 

 when wet, and so fragile when dry, that it can be crushed in the hand. It has a 

 soapy feel and a dull red color. The grain and structure, when examined under the 

 loop, suggest the tuffaceous nature of the soft portions of this rock. These irregular 

 and tortuous variations in the grain and color, and the whole rock, though now 

 feebly cemented into a fragile mass by the prevalent product of its own decay 

 (thalite), seems to have been originally of fragmental origin. The few white amyg- 

 dules seen are of the nature of kaolin, and seem to have resulted from accumulations 



