Exemplified by the Duluth Gabbro. 521 



eroded in the long pre-Keweenawan interval, 9 but could 

 hardly have been stoped into the gabbro, no matter what 

 the horizon of intrusion. It seems certain, therefore, 

 that the gabbro was intruded and spread approximately 

 along the base of the Keweenawan. 



Besides the Duluth mass as a type, one might classify 

 as lopoliths the Sudbury and Bushveldt masses ; and 

 possibly the basin-like mass on the Isle of Skye and the 

 banded rock of Julianahaab, Greenland. 



As a piece of speculation it may be of interest to sug- 

 gest a relation between laccoliths and the larger lopo- 

 liths; and note what would result from a continued 

 increase in size. Figure 2 is self-explanatory. 



General remarks on the Duluth gabbro. — If the form of 

 the Duluth gabbro is as assumed, certain consequences 

 may be stated. The form being roughly lenticular, it 

 seems probable that the extent down the dip is nearly as 

 great as the length of an eroded outcropping edge. Even 

 if it is only half that extent, a glance at the map indicates 

 that it is very probable, as Van Hise and Leith mention, 10 

 that the gabbro of the Gogebic range in Wisconsin is part 

 of the same original lopolith. 



If a roughly circular outline is drawn around all the 

 known outcrops, it encloses over fifteen thousand square 

 miles, the area once occupied by the lopolith; besides 

 which it is evident that a part has been eroded, and prob- 

 able that the subsidence which tilted the gabbro in Wis- 

 consin to an angle of more than 75°, was accompanied by 

 a good deal of crustal shortening. The present area of 

 gabbro outcrops may be much less than the original. 



Estimates of the thickness may be made on the assump- 

 tion that the floor of the gabbro dips approximately with 

 the adjacent internal structure. 11 The estimates are only 

 approximate because of a scarcity of outcrops where the 

 gabbro is widest, and because in the same region there 

 are some thick sills which are distinguished with dif- 

 ficulty from the gabbro. The maximum thickness indi- 

 cated in Minnesota is about 50,000 feet ; at Duluth about 

 12,000 feet are exposed ; at the northeastern outcrops in 

 Minnesota the lopolith is less than 3000 feet thick. These 

 estimates are conservative in the matter of dip, — former 



9 Van Hise, C. R., and Leith, C. K., op. tit., p. 208. 



10 Op. tit., p. 378. 



11 Grout, F. F., op. tit. 



