994 HISTORICAL GEOLOGY. 



The facts reviewed show that the amount of elevation east of the Eocky 

 Mountains, over the northern half of the United States and the adjoining 

 part of British America, increased to the nortliAvard. It is probable that 

 there was a region of maximum height along the Canada watershed south 

 of Hudson Bay ; since the height of a shore-line on James Bay (Mo.) is only 

 450 feet. But this single observation leaves the question doubtful. The 

 general rule of increase to the northward holds over the Winnipeg region, 

 as is shown by the northward rise in the shore-lines of Lake Agassiz ; the 

 upper shore-line, or Herman beach, which at Lake Traverse is 85 feet above 

 this lake, or 1055 feet above sea level, has a height at the national boundary^ 

 224 miles from Lake Traverse, of 1230 feet, and 76 miles farther north, of 

 1315 feet (Upham). 



The heights increased also from the Atlantic coast westward. But there 

 appears to have been a maximum east of Lake Ontario, the heights, as has 

 been stated, diminishing 120 feet along the line of the lake between Water- 

 town at its eastern extremity and its western extremity. The region may 

 have been within the range of the Appalachian uplift of the period as 

 suggested by F. J. H. Merrill. 



How far the change in level extended south of the Great Lakes is doubtful. 

 The small elevation of the shore-line, 45 feet, at the south end of Lake 

 Michigan, indicates nearness to the limit. But south of lakes Ontario and 

 Erie, the distance to the limit may have been two or three hundred miles 

 or more. 



Through these changes, the Arctic, Labrador, Canadian, and New England 

 coasts gained much in extent, and so also some parts of the Pacific border, 

 i!>fova Scotia became again part of the mainland. The beds of rivers flowing 

 south had their pitch increased to its present amount. The river channels 

 within tidal limits were excavated to a deeper level, corresponding more or 

 less closely with the amount of elevation in the region ; and this excavation, 

 as already explained, gave additional height to the bordering terraces. Many 

 lakes were drained that had been made by the northward depression of the 

 land, thus carrying forward the drying of the continent that was commenced 

 with the subsiding of the rivers. 



On the coast of Maine, there are large Indian shell-heaps of the common Clam (Venus 

 mercenaria, the Quahog of the Indians) and, in some places, of the Virginia Oyster, spe- 

 cies which are now nearly extinct on that cold-water coast. As made known by Verrill, 

 there is a colony of living southern species in Quahog Bay, near Bath (20 miles east 

 of Portland), among which are Ve7ius mercenaria Linn., Modiola plicatula Lam., Hya- 

 nassa obsoleta Stimp., Urosalpynx cinerea Stimp., Crepidula fornicata Lam., Asterias 

 arenicola Stimp., Etipagurus longicarpus Edw., and others, reminding one strongly, as 

 Verrill says, of the coast fauna of New Haven, on Long Island Sound. Further, Venus, 

 Uyanassa, Modiola, and other species occur, according to Dawson, also in Northumber- 

 land Straits, in the southern part of the Gulf of St. Lawrence. At the mouth of Dam- 

 ariscotta River, .30 miles east of Portland, there is the only locality of the living oyster 

 north of Massachusetts Bay. Shells of Oysters, Clams, and Scallops (the southern Pecten 

 irradians Lam.) are abundant in the deeper portions of the mud of the harbor of Portland. 



