Page:The American journal of science, series 3, volume 49.djvu/35

Rh Reelevation of the St. Lawrence river basin. 17

with the marine fauna which is preserved in the Leda clays and Saxicava sands.

The Champlain Marine Submergence.

That the land northward from Boston was lower than now while the ice-sheet was being melted away, is proved by the occurrence of fossil mollusks of far northern range, including Leda arctica Gray, which is now found living only in the Arctic seas, preferring localities which receive muddy streams from existing glaciers and from the Greenland ice-sheet. This species is plentiful in the stratified clays resting on the till in the St. Lawrence valley and in New Brunswick and Maine, extending southward to Portsmouth, N. H. But it is known that the land was elevated from this depression to about its present height before the sea here became warm and the southern mollusks, which exist as colonies in the Gulf of St. Lawrence, migrated thither, for these southern species are not included in the extensive lists of the fossil fauna found in the beds overlying the till.

In the St. Lawrence basin these marine deposits reach to the southern end of Lake Champlain, to Ogdensburgh and Brock- ville, and at least to Pembroke and Allumette island, in the Ottawa river, about 75 miles above the city of Ottawa. The isthmus of Chiegnecto, connecting Nova Scotia with New Brunswick, was submerged, and the sea extended 50 to 100 miles up the valleys of the chief rivers of Maine and New Brunswick. The uplift of this region from the Champlain sea level was 10 to 25 feet in the vicinity of Boston and northeast- ward to Cape Ann ; about 150 feet near Portsmouth, N. H. ; from 150 to about 300 feet along the coast of Maine and southern New Brunswick ; about 40 feet on the northwestern shore of Nova Scotia ; thence increasing westward to 200 feet in the Bay of Chaleurs, 375 feet in the St. Lawrence valley opposite the Saguenay, and about 560 feet at Montreal ; 150 to 400 or 500 feet, increasing from south to north, along the basin of Lake Champlain ; about 275 feet at Ogdensburgh, and 450 feet near the city of Ottawa. The differential elevation was practically completed, as we have seen from the boreal charac- ter of the Champlain marine molluscan fauna, shortly after the departure of the ice sheet. With the areas of the glacial Lakes Agassiz, Warren, and Iroquois, in the interior of the continent, this coastal region gives testimony of a wave-like epeirogenic elevation of the formerly ice-laden portion of the earth's crust, proportionate with the glacial melting and closely following the retreat of the ice from its boundaries of greatest

Am. Jour. Sci.— Third Series, Vol. XLIX, No. 289.— Jan., 1895. 2