522 Grout — Lopolith; An Igneous Form. 



records of structure would indicate nearly twice as steep 

 a dip as that here used. 12 In Wisconsin the thickness of 

 the gabbro is probably less than 4,000 feet, 13 If the lopo- 

 lith is thickest in the center like a lens, the real maxi- 

 mum thickness is concealed below the lake. 



The volume of the lopolith may be estimated at over 

 50,000 cubic miles. It is evidently one of the largest 

 known intrusive masses. Considered with some related 

 intrusions, — the Logan sills, the sills at Beaver Bay, and 

 other intrusions of the same age in more distant parts of 

 the Lake Superior region — it indicates an immensity of 

 intrusive action at this time, that has rarely been 

 equalled. 



Summary. — Certain large, centrally sunken intrusions 

 are given a distinct name, lopolith. Lopoliths differ from 

 laccoliths not only in these points of size and form, but 

 probably also in the mechanics of their intrusion. The 

 Duluth gabbro is a multiple, composite, divided lopolith 

 which is furthermore interformational over most of its 

 length. Conservative estimates of its size indicate an 

 area of over 15,000 square miles, and a volume of over 

 50,000 cubic miles — one of the largest known floored 

 intrusions. Other illustrations of lopoliths are sug- 

 gested. 



Acknowledgments are here gratefully given to the 

 members of the geologic staff of the graduate faculty at 

 Yale University, for very helpful suggestions. 



12 Irving, E. D., op. tit., p. 269. 



13 Van Hise, and Leith, op. cit., p. 377. 



