STRUCTURAL GEOLOGY. 73 



The Manitou epoch of eruption.] 



westward to the western confines of the Mesabi Iron range. Here the country is 

 also much drifted, but it can be assumed that the same formation is practically 

 continuous at the surface as far as the central and western part of Carlton county, 

 and to the western part of Morrison county, where it is again well known. Whether 

 in some parts of this drifted area it is covered by the Animikie, is immaterial. 



Toward the northeast, at and near Gunflint lake, where the Animikie again 

 appears distinct from the muscovadyte and the gabbro, the Animikie lies on granite, 

 and this relation continues northeastwardly, according to Mr. E. D. Ingall, for many 

 miles into Canada. Hence, if the greenstone belt above mentioned continues further 

 east, it must run below the Animikie along the south side of this granitic belt. 

 Toward the east, north and northwest, therefore, it is fair to assume that the gabbro 

 is in intimate relations with a greenstone belt. Toward the west it is covered by 

 drift and the nature of the rock is unknown, but is probably the same as far as to 

 western Carlton county. Toward the south and east the older rocks are hid by the 

 Keweenawan. This is sufficient to show that, in all probability, the crescentic line 

 of folding and of metamorphism which outlines roughly the gabbro area in Minne- 

 sota, intrenched on a prior existing belt of greenstone which seems, toward the north- 

 east, to have passed below the Animikie but caused the penetration of the Animikie 

 by many great sills and dikes, and toward the southwest, as seen about Carlton and 

 Cloquet, to have sent similar dikes into the contiguous formation. 



If the greenstone member of the Archeau, the oldest known rock, be the source 

 of the gabbro, and the gabbro be the source of the sills and dikes of the Animikie, an 

 important corollary can be drawn. The greenstones, underlying the gneisses of the 

 Archean and being older than any of the granites intrusive into them, are capable of 

 furnishing, on any similar occasions of refusion throughout geologic history, not 

 only bosses of gabbro, but dikes of diabase, even to the present time. 



The IfiniitoH epoch of eruption. After the removal of the Beaver Bay diabase 

 from the body of the gabbro there appears to have been a period of tumultuous 

 oceanic transportation during which was formed the Puckwunge conglomerate. 

 For a short time the igneous forces were comparatively still, allowing the accumula- 

 tion, especially on the south side of lake Superior, of great thicknesses of coarse 

 conglomerate. But even during the age of this conglomerate, and especially during 

 the formation of the sandstones that followed it, there were occasional and local 

 lava flows which must have extended for many miles. After each epoch of these 

 later eruptions the lava sheet was covered by a sandstone, often conglomeratic, 

 derived principally from the disintegration of the previous trap sheet. This suc- 

 cession of igneous and fragmental rocks characterizes the Lower Keweenawan of 

 Irving. It is apparently much thicker on the south side of lake Superior than on 



