520 



Grout — Lopolith; An Igneous For 



m 



find the roof at a fairly constant horizon. The magma 

 must have spread along an unconformity, or we are 

 forced to the absurd conclusion that the magma knew 

 when to cease its stoping. Another fatal objection to the 

 idea of transgression and much stoping, is the volume of 

 material missing. The Rove slate, where it dips under 



Fig. 2. 



Fig. 2. Sketches to suggest the possible relation of a lopolith to the other 

 forms of occurrence of igneous rocks. 



the gabbro, is estimated to be 2600 feet thick, 7 and this is 

 only one of several missing formations. Hall has esti- 

 mated that the slates west of the gabbro are 20,000 feet 

 thick, 5000 feet in sight. 8 The estimates are not based on 

 accurate data, but are probably of the right order of 

 magnitude. These formations could easily have been 



7 Van Hise, C. E., and Leith, C. K., Geology of the Lake Superior region; 

 U. S. Geol. Survey, Mon. 52, p. 201. 



8 Hall, C. W., The Kewatin of eastern Minnesota: Bull. Geol. Soc. 

 America, vol. 12, p. 374, 1901. 



