636 FRANK F. GROUT 



Petrographically there is little that is new to the Keweenawan. The 

 greater part of the rock originally occurred in surface flows, with 

 an occasional conglomerate apparently formed along some shore- 

 line. Beds of tuff, derived from volcanic ash and breccias, are not 

 numerous, but are prominent in a few places. Intrusive dikes were 

 not identified in this area. Evidence of the effusive nature of the 

 rocks lies not so much in flowage structures as in the regular sequence 

 of textures — a compact aphanitic basal portion grading into the 



Fig. 2. — Ditch at Pine City, Minn., showing lava Hows dipping 67°. Flows may- 

 be distinguished by the changes of the jointing in the amygdaloid. 



coarser body of the flow, with often a characteristic amygdaloid 

 breccia or ash bed before the next flow repeats the series. The 

 jointing is in most places irregular, but in some columnar, as, for 

 example, that producing the Devil's Chair at the Dalles of the St. 

 Croix. The main jointing seldom persists into the amygdaloid, thus 

 giving another means of distinguishing successive flows (see Fig. 2). 

 The chief evidence of faulting is the development of slickensides in 

 several places, and these may be along very minor fractures due to 

 the extensive tilting of so thick a formation. There are no signs of 

 the erratic discontinuance of a flow, or a shifting of position. 

 Spheroidal weathering is quite common. Exceptional structures 



