434 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



with them. On the north, along the great Lanrentine ridge, they 

 rest unconformably on a series of metamorphic rocks, older than 

 those just spoken of, and consisting of syenitic gneiss, white marble, 

 hornblende-rocks, chloritic and other slates, largely intermixed with 

 phosphate of lime. 



But there is, on the north shore of Lake Huron, and on both sides 

 of Lake Superior, 600-1000 miles to the west of central New York, 

 an extensive suite of rocks, which stand in the same relation to the 

 Silurian beds as do the Longmynd of Wales. This suite has been 

 carefully examined by Sir William Logan*, Messrs. Murray, Whitney, 

 and Foster. The quasi-Longmynd of Lake Huron is at the same 

 time unconformable to the Silurians above them and the old meta- 

 morphic rocks below ; its total thickness is about 10,000 feet ; and it 

 extends about 80 miles along the shore of the Lake. 



This formation is lithologically different from the Longmynd of 

 Wales, — being of white, often vitreous sandstone, with many beds of 

 quartz-conglomerate ; and it alternates with great masses of green- 

 stone-slate, and of conglomerate with pebbles of syenite and granite 

 in a matrix of greenstone. A persistent band of limestone, 150 feet 

 thick, with thin layers of chert, is found in the middle of the series. 

 Greenstone-dykes, large and small, everywhere traverse the strike in 

 considerable numbers. 



The rocks upon which the Huron Longmynd rest are micaceous 

 and chloritic schists, gneiss, and granites, — a continuation almost 

 certainly of the older metamorphic rocks in more eastern Canada. 



It will be convenient for the sake of brevity simply to refer to 

 Foster and Whitney's Report for information respecting these oldest 

 rocks on the south side of Lake Superior, and to Silliman's Journal 

 (n. s. vol. xiv. p. 227) for the N.E. portion of that Lake. 



The lower stage of the Silurian system is distinguished from the 

 others by the four following important features, besides a multitude 

 of minor details, as discovered by a comparison, section by section. 



1. The groups of the lower stage are the most extensive. They 

 are in sheets 1000 and 1400 miles in diameter. We have Calciferous 

 Sandstone and Trenton Limestone in the Gulf of St. Lawrence, and 

 stretching westwards and southwards through the Canadas, New 

 York, into Michigan, Wisconsin, Minnesota, Texas, &c. Neither 

 are the other groups very inferior in extent. 



2. It is remarkable for the comparative uniformity of mineral 

 condition of the same section in very distant countries. Variations 

 occur frequently, but there is a constant recurrence to the normal 

 mineral type. The Potsdam Sandstone of the Mississipi or of Lake 

 Superior is described in nearly the same terms as that from eastern 

 New York. The difference consists in the proportion of clay or 

 oxide of iron contained. The same may be said of the other strata. 

 The steady, regulated mineral change westward, observed in the 

 middle and upper Silurians from east to west, has not commenced, — 

 a very noticeable fact. 



* Sill. Journ., n. s. vol. xiv. p. 225. 



