OF THE RIVER SAINT LAWRENCE. 



253 



drain back valleys, it skirts the great plain, and enters the State of Vermont, which it 

 traverses under the name of the Green Mountains, three thousand feet high. It is con- 

 tinued as the Berkshire Hills in Western Massachusetts, and as the 'laconic Hills in Eastern 

 New York; crosses the Hudson as the Highlands of West Point, and the Delaware as the 

 Durham, or Easton Hills. Lost for a few miles between the Schuylkill and the Susque- 

 hanna at Harrisburg, it re-cmcrges from beneath the New lied plain as the chain of the 

 South Mountains of Southern Pennsylvania,. In Maryland, it crosses the Potomac at Har- 

 per's Ferry, to form the Blue Ridge of Virginia, and the Smoky Mountains which divide 

 Tennessee from North Carolina; where the Black Mountain group, a little east of the line, 

 attains elevations ranging between six and seven thousand feet above the sea. Traversing 

 Georgia, the chain sinks beneath the Cretaceous plain of Middle Alabama, and is seen no 

 more, after having a geographical range of not less than sixteen hundred miles. 



The geological cause for the; shape and position of the estuary and lower river of the 

 St. Lawrence must not be overlooked. It is to be found in the presence of a remarkable 

 fault or fissure in the crust of the earth, running close along the southern shore from Gaspe 

 to Quebec, thence through the middle; of the plain up the east shore of Lake Champlain, 

 and down the Hudson River into New Jersey. All the rock formations on the northern 

 and western side of this fault, both in Canada and in New York, are thrown down to a 

 depth varying from five to ten thousand feet. The top of the lower Silurian system in the 

 west wall of the fault, is brought down to a level with the bottom of the same system in 

 the east wall. In these soft Hudson River slates, as they are called, have therefore been 

 excavated, all along on the west side of the fault, the estuary of the St. Lawrence, the 

 Lakes St. Peter and Champlain, and the Hudson River valley; for the same agency brings 

 abruptly to an end in the Catskill Mountain, three; thousand feet high, on the west bank 

 of the Hudson Riven - , the Alleghany Mountain system coming up from the southwest 

 through Middle and Northern Pennsylvania. 



Passing now to a description of the basin of Lake Ontario, its limits are of quite another 

 order. Its eastern end abuts against the Laurentian rocks of the; Adirondack Mountains 

 of New York, and its outlet is over the low and narrow barrier of tin; same forming the 

 Thousand Islands. The lake itself is excavated out of the soft lower Silurian rocks de- 

 scribed. The northern limit of the basin is an east and west line, about fifty miles back 

 from the northern shore; the western continuation of the Laurentide Mountains in their 

 course from the Thousand Islands to the foot of Lake Simcoe. Its southern limit is made 

 by three remarkable escarpments, ranging in parallel east and west lines from the Hudson 

 River to Lake Eric, caused by the broad outspread and almost imperceptible southern dip 

 of the whole Palaeozoic system, from the Potsdam sandstone at the bottom of the Silurian, 

 to the coal beds at the bottom of tin; Carboniferous rocks. This dip being towards the 



