THE BASIC MASSIVE ROCKS, ETC. 695 



to the Animikie and the Keweenawan rocks. Some of the rocks, 

 called by Winchell Pewabic quartzite, are probably true Animikie 

 fragmentals, or metamorphosed phases of these, but even in this 

 case there is no proof that the gabbro immediately_succeeds them 

 in point of age. The evidence would simply indicate that the 

 eruptive is younger than the Animikie. It would not fix its age 

 more definitely. The observations of Winchell would thus seem 

 to lead to the same conclusion as that reached by Irving in so 

 far as the latter supposed the gabbro to be post-Huronian. 



Upon returning again to the problem as to the age of the 

 gabbro Winchell J attempts to fix this more definitely by assum- 

 ing the identity of this rock with the anorthosite, which is shown 

 by Lawson to be older than the bedded Keweenawan. But it is 

 impossible at present to assert with any degree of certainty, that 

 the two rocks are the same (although VanHise holds with Win- 

 chell that their equivalency is possible), for the one has not been 

 traced into the other, nor has the upper limit of the gabbro been 

 carefully studied. This great mass may be much older than the 

 lowermost beds of the Keweenawan series, but as yet there has 

 been cited no proof in favor of the view. 



So far as the little evidence at hand enables us to judge, the 

 gabbro whose petrographical characteristics are discussed in this 

 article, forms a great mass of enormous extent above the Animikie 

 but below the interbedded flows and fragmentals of the Kewee- 

 nawan series in Minnesota. There are obscure indications that 

 the mass is a great layer composed of successive ■ flows that fol- 

 lowed one another so rapidly as to give no opportunity for the 

 action of erosion processes or for deposition between them. If 

 this be so the lack of more apparent bedding is doubtless due to 

 the great thickness of the individual beds, as is also their coarse 

 grain. There are some things about the mass, however, that 

 suggest another origin for it. "The great coarseness of grain, 

 the perfection of the crystallization, the abrupt termination of the 

 belts, the complete want of structure, and the presence of inter- 

 secting areas of crystalline granitoid rocks — all suggest the 

 'Bull. No. 8. Geol. and Nat. Hist. Survey of Minn. p. xviii. 



