Dr. J. W. Spencer' — Subsidence 'versus Glacial Bams. 265 



Carboniferous sandstones, and it is out of this material that the 

 pebbles were formed. 



There are similar superficial gravels on other, but, of course, 

 inferior knobs along the very foremost portions of the " terminal 

 moraine." But the drainage from these ridges is to the nortli, 

 and Mr. Lewis emphasized the fact that there is no drift in the 

 small streams flowing to the south. The theoretical importance of 

 this observation "will be noted later. 



Besides these highest of all the superficial gravels, south of the 

 Great Lakes, which I have examined, I have also visited the high 

 terraces of the Genesee Kiver flowing northward from the deposits 

 just described. Here several pauses in the receding waters are 

 recorded. These are notable from an elevation of 1900 feet down- 

 ward. At this named high altitude the valley is nearly a mile 

 wide, and now 250 feet below the terrace. Our knowledge of 

 these elevated and disconnected water deposits is yet very scanty, 

 but certainly very suggestive, when supplementing the surveys 

 of the lower coast-markings in the lake region. 



A very interesting terrace remains in a valley three or four miles 

 to the east of Horseheads, New York. The altitude of the terrace is 

 1200 feet above tide, whilst the gravel-covered floor of the valley, 

 at Horseheads, is only 900 feet. This last vallej'^ is over a mile 

 wide, and is that connectina; the trough of Seneca Lake with the 

 Susquehanna Valley. 



Similar elevated terraces have been noted by Prof. I. C White 

 along the higher Potomac Valley facing the Atlantic, and along the 

 adjacent tributaries of the Monagahela, which drains to the west- 

 ward. These deposits he notes up to an elevation of 1675 feet 

 above the sea, and 175 feet above the valley, along a tributary creek 

 above St. George, W. Va.^ 



At Nachvak, in Labrador, Dr. Robert Bell found beaches of great 

 distinctness at 1500 feet above the sea. Gravel and shingle terraces 

 were also found to an estimated height of 2000 feet.^ 



It has already been noted that the differential rise of the L'oquois 

 Beach, north of the Adirondack Mountains, amounts to six feet per 

 mile, and that it has there been lifted to a thousand feet. If this 

 rise continue to the White Mountains, then the equivalent of the 

 Iroquois Beach may be found amongst the terraces of the high 

 valleys in that region. Its records may be preserved still further 

 north-east, on the drift-covered sides of Mount Katahdin, in Maine. 

 Mount Desert, on the coast of Maine, rises to 1500 feet,^ and shows 

 remnants of coast action to its summit (Shaler) ; consequently, it is 

 too low to bear records of the Iroquois shore, unless the warping at 

 the earth's crust becomes one of depression east of the Adirondacks. 



In Ontario, some of the high shores, referred to above, occur at 



1 "Rounded Boulders at High Altitudes," by I. C. White, Amer. Journ. Sc. 

 vol. xxxviii. 1887. 



^ Eept. Geol. Surv. Canada, 1885, DD, p. 8, and Bull. Geol. Soc. Amer. 

 vol. i. p. 308. 



^ "Geol. Mount Desert," by N. S. Shaler, Eept. United States Geol. Survey, 

 1888, p. 993. 



