XVin INTEODUOTION. 



a laro-e })art of the latter is cjmposed of volcanic material. It is not likely 

 that the sediments at any one place are as much as 5,000 feet thick. 



The Upper Huronian is a great slate and schist series, which it is not 

 possible to separate on the maps into individual formations, and it is impos- 

 sible to give even an approximate estimate of the thickness of this series. 



Various igneous rocks intrude in an intricate manner both the Upper 

 Huronian and the Lower Huronian series. 



The aim of the following paragraphs is to sketch very briefly the 

 history of the district. 



THE ABCHEAN. 



The Archean consists mainly of massive and schistose gi-anites and of 

 gneisses. Nowhere in the Archean have any rocks of sedimentary origin 

 been discovered. The Archean has been cut by various igneous rocks, 

 both basic and acid, at different epochs. These occur in the form both of 

 bosses and of dikes, the latter sometimes cutting, but more ordinarily show- 

 ing a parallelism to, the foliation of the schistose granites. The granites 

 must have formed far below the surface, and therefore must have been 

 deeply denuded before the transgression of the Lower Huronian sea. The 

 Archean granites and gneisses and th.e earlier intrusives alike have been 

 profoundl}^ metamorphosed, and at various places have been completely 

 re crystallized. 



THE LOWER HURO^TIAN SERIES. 



The Sturgeon quartzite, the first deposit of the advancing sea, when 

 formed consisted mainly of sandstone, but in ])laces at the base of coarse 

 conglomerate. The conglomerate is best seen in the Sturgeon River 

 tongue. Elsewhere e^^dence of conglomeratic character at the base of 

 the formation is seen, but the metamorphism has been so great as nearly to 

 destroy the pebbles. However, in the Sturgeon River tongue is a great 

 schistose conglomerate, which, while profoundly metamorphosed, still gives 

 evidence of the derivation of its material from the older Archean rocks. 

 The sandstone has been changed to a vitreous, largel)^ recrystallized 

 quartzite, which now shows only here and there vague evidence of its 

 clastic character. 



The Sturgeon formation varies from probabl)- more than 1,000 feet in 



