56 THE GEOLOGY OP MINNESOTA. 



[Extension of the gabbro toward the east. 



own internal structure and composition, as well as in the number of petrographic 

 principles illustrated, the gabbro surpasses any other member of the Taconic, and 

 perhaps equals in interest the granites of the Archean. Being an igneous rock of 

 the opposite end of the magmatic scale it offers a new set of petrographical principles, 

 and, with its attendant accidents of contact, exceptions and variations, it serves to 

 add a large chapter to the petrographic geology of the state. 



The numerous problems arising from the gabbro cannot all be considered as 

 solved by the examinations the survey has given to them. There is a large uncropped 

 field for the future student to enter upon. The structural discussion which follows 

 is a tentative effort to set forth and to explain the leading facts of the gabbro on a 

 general hypothesis, which is at the same time supported by all the special facts 

 which are in our possession, or at least is not known to contravene them. 



The extension of the gabbro toward the cast. The eastern extension of the gabbro 

 mass has sometimes been limited in the vicinity of East Greenwood lake, T. 64-2 E., 

 and that approximately expresses the eastern limit of the continuous surface outcrop 

 of the great body of the gabbro. At a short distance east of this lake it becomes 

 covered by later diabasic flows, or loses itself in a series of sills and dikes that 

 penetrate the Animikie. From Gunflint lake to Pigeon point the Animikie is thus 

 affected. In some cases the body of the gabbro comes within a few hundred feet 

 of the most southern mass of the Animikie bluffs, the Animikie having a dip southerly 

 so as to throw it below the gabbro. This is particularly the case at Loon lake (south 

 of Gunflint lake), where coarse characteristic gabbro is within 150 feet of the Ani- 

 mikie bluff, and the irresistible inference is that the great sills of diabase which are then 

 seen in the Animikie at lower levels, and which are themselves sometimes interme- 

 diate petrographically between gabbro and diabase, are only apophyses from the 

 gabbro. Such sills are of all dimensions, from one or two feet to fifty and more feet in 

 thickness. Owing to the low dip of the Animikie, and to its easy removal by natural 

 causes, it is seldom seen to persist on the tops of these mono-clinal hills, but almost 

 everywhere the usual surface rock is that of one of these sills. This diabase-gabbro, 

 therefore, might be said, in general, to extend much further east and north than the 

 limit usually assigned to the gabbro itself. Indeed, it is very probable that even 

 .within the gabbro body, along the northern limit of surface continuity, there exists still 

 intact much of the Animikie, and that such beds would be found on sinking a drill 

 through the gabbro. Where these great sills constitute the representative of the 

 gabbro, as at Rove lake, Mountain lake, Fowl lake and eastward into Canada, the 

 country is generally very much broken, the mono-clinal hills and ridges rising steeply 

 from 200 to 400 feet from the valleys on the north, but sloping more gently from 

 their summits to the valleys on the south, the topography being due to the gentle 



