410 THE VERMILION IRON-BEARING DISTRICT. 



8[7]. The bottom of the sedimentary strata above them, wherever it is observable, 

 is a freshljr raptured surface. 



9[8]. Apophyses of the trap pass from the main sheet into the cracks of the slate 

 above and below. 



10[9]. The trap sheets, particularly at the upper contact, hold included fragments 

 of the overlying- slates. 



11[10]. They locally alter the slates above and below them." 

 The writer does not believe that the faulting sug-gested in I, 2, above, 

 explains the structure of the rocks, as no evidence in favor of this faulting 

 has been observed. Facts have been observed in the Lake Superior region 

 which are corroborative of Lawson's other statements in all their essentials, 

 and confirm his conclusions. 



Relations of sills to Keweenaivan. — Lawson'' goes still further, and draws 

 the important conclusion from evidence observed during his field work that 

 these sills are not only later than the Animikie sediments, but are intrusive 

 in the Keweenawan, and states it as his opinion that the)^ are identical in 

 age with many of the heavy sheets of dark diabase or gabbro which prevail 

 on the Minnesota coast, particularly in the eastern portion. He therefore 

 places the sills as post-Keweenawan, and possibly of Silurian age. The 

 writer dissents altogether from the idea that these sills and dikes can be 

 post-Keweenawan, since nowhere in the Lake Superior region have the 

 Cambrian rocks been found to be cut by dolerite sills and dikes, or indeed 

 by any intrusives. While the sills may be younger than part of the 

 Keweenawan, there is no reason for supposing them younger than all of 

 the Keweenawan, but they may belong to the same period of igneous 

 activity and be merely one of the later expressions of this activity, when 

 the molten magma was too deeply bui-ied to reach the surface freely as 

 flows, and was intruded between the sediments of the Animikie and the 

 lava sheets of the Keweenawan wherever conditions were favorable. Thus 

 considered they would be of Keweenawan age instead of post-Keweenawan, 

 as has been supposed by Lawson. 



Relations of the gabhro to the Logan sills. — The question of the relation- 

 ship of the gabbro to the Logan sills has been discussed by Grant, who 

 states that he "is inclined to separate the sills from the gabbro, but admits 

 that this separation is not proven." The various facts which he considers 



a Lawson, loc. cit., pp. 44-45. 



b Geol. and Nat. Hist. Survey of Minnesota, Bull. No. 8, 1893, p. 47. 



