186 PHYSICAL GEOGRAPHY. 



forma! ion is much cut by greenstone dikes, and exhibits 

 very frequently interstratified beds of greenstone, often of 

 great thickness. Both these and the sedimentary rocks 

 contain metalliferous quartz veins, of which the copper 

 mines of this region are examples. Resting uncomform- 

 ably on the tilted edges of this formation, and in other 

 places directly upon the southern limit of the syenitic 

 gneiss, appear the silurian rocks, identical with those which 

 are found in New York, and covering the peninsula be- 

 tween Lake Huron and Lake Ontario. Beginning with 

 the Potsdam sandstone, we have, upon the Manatoulin 

 Islands and the coast between Matchedash Bay and Sarnia, 

 a complete exposure of those formations known as the 

 Trenton limestone, Utica slates, Loraine shales, Medina 

 sandstones, and the Niagara limestones, with the rocks of 

 the Clinton group. All these are well characterised by 

 their respective fossils, and are spread out quite undis- 

 turbed at a very gentle dip of about thirty-five feet in a 

 mile. 



" Passing to the east, we find that the syenitic rocks 

 have divided where they cross the Ottawa, and, taking a 

 southward course, are spread over a considerable extent 

 of country between the Ottawa and the St. Lawrence. 

 Crossing this river below Kingston, they constitute the 

 greater part of the Thousand Isles *, and are extensively 

 developed in the northern counties of New York." 



Mr. Logan gives a more particular account of the north 



* In the Lake of the Thousand Isles, as the funnel-shaped outlet 

 of Lake Ontario is denominated, many of the round-backed hummocks 

 of granite which form the innumerable islets exhibit the parallel 

 furrows, streaks, and smooth surfaces attributed by some geologists 

 to glacial action. This expansion, in fact, has exactly the aspect of 

 many of the dilatations of the northern rivers which flow through the 

 " intermediate primitive district." 



