LITHOLOGY OF THE SERIES 367 



microscope, seemed to be freest from impurities, gave the following 

 result : 



Per cent. 



SiOa-. 5.02 



AlA 14.20 



Fe.A *. 2.00 



CaCOs 60.52 



MgCOs 19.11 



100.85 



Farther to the north, in the midst of the graywacke slates, the horn- 

 blendic graywackes, and the hornblende schists, bands of carbonates 

 occur, varying from a few inches to several feet in thickness. They show 

 numerous grains of quartz intermingled with the limestone. In short, 

 the same characters, save in color and degree of alteration, distinguish 

 these bands as have been described for the thoroughly crystalline lime- 

 stones near Sturgeon lake. 



THE GRANITES 



These rocks are chiefly of the hornblende-biotite type. In Morrison 

 country a light gray biotitic, rather finely textured outcrop occurs. At 

 the now abandoned site of Granite City an unusual hornblende gneiss 

 occurs. Gneissic features occur along the Rum river. At two or three 

 localities on Snake river interesting dikes of granite break through the 

 schists and make them more coarsely crystalline. 



For miles along the Mississippi river, past Watab, Sauk Rapids, and 

 Saint Cloud, exposures of these granitic rocks abound. Of the horn- 

 blende-biotite type, they once were augitic rocks, for in the freshest ex- 

 posures augite cores still remain in the midst of the clustered hornblende 

 individuals, while, as the rule of distribution, biotite individuals form 

 a circle outside the hornblende clusters. 



THE DIABASES 



The dike rocks, of which two or three varieties are included under the 

 more generic term diabase, occur throughout the entire district under 

 discussion. They present some local phases of interest, and show a 

 considerable range of special characters. They can not here be described 

 in detail. It may be said that, as a rule, they are of the porphyritic 

 type. Feldspar is usually in lath-shaped individuals, lying within a 

 groundmass of feebly reacting minerals, finely crystalline in texture and 

 to a great extent altered from their original condition. The feldspars 

 are in some instances, as in a dike in the railway cut south of Carlton, 

 extremely fresh crystals of labradorite, and in others so far altered into 



LII— Bull. Geol, Soc. Am., Vol. 12, 1900 



