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ON THE HYDROLOGY OF THE BASIN 



plain of almost horizontal lower Silurian rocks, covered with variable depths of post-tertiary 

 (days, showing themselves in remarkable terraces around the border lines, at an average 

 elevation of two hundred feet above the plain. Five or six masses of trap form as many 

 isolated mountains, from six hundred to twelve hundred feet high, standing upon the plain 

 like stranded ships upon a beach. One of these is the mountain of Mont lloyale. Around 

 these island-mountains the terraces of post-tertiary clay are visible. The plain, however, 

 is not confined to the triangular space between the two great rivers ; it spreads on east- 

 ward, past the Richelieu and Lake Champlain, to the foot of the Green Mountains, next 

 to be described, and so along the range past the Yamaska and St. Francis, in a long and 

 narrow belt, even to Quebec. The whole area of this plain thus described contains about 

 eighteen hundred square miles, the most of which is fertile arable land, well watered and 

 level, through the midst of which flow the two great rivers named* 



The only important adjunct of the basin of the St. Lawrence proper is to be found in 

 the extension of this lower Silurian plain southward up the Vermont, or eastern shore of 

 Lake Champlain, the western, or New York shore of which rises at once into the Adiron- 

 dack heights. Starting from the head of Lake Champlain, a narrow winding gorge 

 between high mountains of Laurentian rocks terminates in Lake George (so famous among 

 tourists), the upper end of which is separated only by seven miles from one of the principal 

 head rivers of the Hudson, the difference of elevation, however, in favor of Schroen lliver 

 being at least five hundred feet. 



We must now describe the eastern portion of the southern barrier of the St. Lawrence 

 basin. It has no connection whatever with the western portion already described. Lake 

 Champlain, with its side plain of lower Silurian rocks, opening up a great highway between 

 Canada and the Atlantic States, isolates the Adirondack Mountains on its western shore 

 from the Green Mountain range, from which its eastern affluents descend. Unlike; the 

 Laurentide Mountains on the north, the southern limit of the basin is a corrugated 

 plateau or chain of parallel ridges of quartzite, slate, and limestone rocks, of lower Silurian 

 age, about fifteen hundred feet high, upon the top of which rise to a still loftier elevation 

 the Schick-Shock, and other isolated groups of synclinal lower Silurian mountains, proba- 

 bly connected geologically with the Katahdin Mountains of Maine, and the White Moun- 

 tains of New Hampshire. Commencing at Cape Gaspe, this barrier ranges along the 

 southern shore of the Gulf and Fstuary, in a graceful curve, three hundred miles, to the 

 neighborhood of Quebec, where it leaves the river, by slowly diminishing the radius of its 

 curve, towards the south. Crossing the Chaudiere and St. Francis, the waters of which 



* A view of this plain, looking southeast, bounded by the mountains just described, from the mountain of 

 Montreal, accompanied the memoir. — l'uii. Com. 



