640 FRANK F. GROUT 



The breccia consists of angular fragments of amygdaloid suspended 

 in a fine-grained fragmental matrix identical with the tuff; and not, 

 as suggested by Dr. Berkey, suspended in the flowing lava or cemented 

 by igneous material. The tuff is fine grained in most places, and the 

 fragments angular; though rounded grains and water-sorting are 

 not difficult to find. Alteration has produced a hard mass of quartz 

 and epidote, an almost indestructible rock. Tuffs and breccias 

 mark the boundary zone between two flows, though in a few cases 

 fragments have been suspended in a later flow. 



Types of Igneous Rocks. — The rock-types described and 

 pictured by earlier writers on the Keweenawan are not all repre- 

 sented in the lavas of eastern Minnesota. In fact, so few are repre- 

 sented that it is unnecessary to use as elaborate a table as A. N. 

 Winchell presents in correlation of the early work. Of his types, 

 based on modern petrographic usage, the area furnishes diabase and 

 olivine diabase with some approaches to basalt and augite andesite, 

 and the corresponding porphyries. In all the sections examined the 

 essential of diabasic or ophitic texture was observed, viz., the plagioclase 

 needles formed before the augite. This approaches a granular texture 

 when augite is neither abundant nor coarse grained, but if the funda- 

 mental fact be kept in mind, all may be classed as modifications of 

 diabase. No distinction is here drawn between augite and diallage; 

 the presence or absence of olivine is of secondary importance like 

 that of magnetite. The variation of the rock (or its alteration) is 

 evidenced by the range in silica from 42 to 65 per cent. If reference 

 is limited to this Minnesota area, the varieties are more clearly 

 classified by varying texture than by the presence of olivine or other 

 characters suggested by Mr. Winchell. The prevalent diabase is a 

 quite uniform, compact rock with hackly fracture, coarse to fine 

 grained (Fig. 4) , grading, on one hand, by all degrees into a rock that 

 is strongly luster-mottled (Fig. 5), also coarse to fine; and on the 

 other hand, into a rock with clearly conchoidal fracture (Fig. 6), 

 often red in color and usually fine grained. These field distinctions 

 correspond to microscopic variations given below. They are to be 

 correlated with Irving's "ordinary," "olivinitic," and "ash-bed" 

 diabase, respectively, but are all included in Winchell's diabase and 

 olivine diabase. All are easily accessible near Pine City, Minnesota. 



