THE BASIC MASSIVE ROCKS, ETC. 693 



belonging with the Keweenawan rocks, the Wisconsin mass was 

 nevertheless recognized by Irving as presenting "the appearance 

 of a certain sort of unconformity with the overlying beds. These 

 gabbros, which lie immediately upon the Huronian_ slates, form a 

 belt which tapers out rapidly at both ends, and seems to lie right 

 in the course of the diabase belts to the east and west, since these 

 belts, both westward toward Lake Numakagon, and eastward to- 

 ward the Montreal river, lie directly against the older rocks, with- 

 out any of the coarse gabbro intervening." . . . "The great 

 extent of coarse gabbro in Minnesota seems to sustain somewhat 

 the same relations to more regularly bedded portions of the 

 series." T 



The only other descriptions of this great gabbro mass are to 

 be found in the reports of the Minnesota survey. In the report 

 for 1887 Prof. N. H. Winchell 2 details a few of his observations 

 on the "great gabbro flood," and surmises that the "flow" did 

 not escape through a single fissure. The structure of the rock is 

 reported as roughly columnar, with sometimes apparent indica- 

 tions "of the existence of imbricating layers having a gentle dip, 

 as if the fluid rock had swept over the country in successive 

 tides. ... In texture the gabbro is characteristically coarse. 

 Sometimes some of the constituent minerals are half an inch in 

 diameter. From this they graduate down to an extreme degree 

 of fineness." 



From the macroscopic descriptions of other varieties of the 

 rock that follow it is evident that the writer is not dealing exclu- 

 sively with specimens taken from the great "gabbro flood" at the 

 base of the Keweenawan, for, as the sequel will show, this 

 is composed of a rock which, in its unaltered state, possesses a 

 remarkably uniform texture, and is so well characterized that any 

 departure from it is presumptive evidence that the rock exhibit- 

 ing the variation belongs not in the "basal flow," but in some 

 one of the numerous smaller beds interstratified with the Animi- 



1 Copper-Bearing Rocks, p. 1 55- 



2 Geol. and Nat. Hist. Survey of Minnesota, 16th Ann. Rept. for 1887. St. Paul, 

 1888, pp. 360-362. 



