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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Apr. 10, 



as the traveller follows a westerly course, by an abrupt rise amounting 

 to 600 feet. They have a very irregular outline, and are cut through 

 by the river in many cases, so as to form isolated masses of broken 

 table-land. (See general section : Sections Nos. 1 & 2, PI. XIII.) 



The Rocky Mountains forming the western limits of the great 

 plateau rise from it very abruptly, the eastern ranges often present- 

 ing sheer cliffs 2000 to 3000 feet in height. These are, however, 

 cut by transverse valleys into which the superficial deposits of the 

 prairies penetrate, and have been preserved, more or less perfectly, 

 as terraces in the mountain-valleys. 



The mountains formed of broken plications of strata, as will be 

 afterwards described, are disposed in parallel groups, the great valleys 

 in the length of the chain generally occupying anticlinal fractures. 

 The flexures have been more perfectly developed in the eastern part 

 of the chain than towards the central parts, where the mountains have 

 a massive cubical aspect, the strata having been fractured and up- 

 heaved rather than bent by disturbing agencies. This is owing, 

 no doubt, to the mineral composition of the strata, and not to any modi- 

 fication of the disturbing force ; for, as the western slope is descended, 

 slaty rocks are met with, which again present perfect flexures. The 

 mean altitude of the Rocky Mountains between latitude 49° and 

 53° is about 12,000 feet above the sea, but there is a very singular 

 absence of marked peaks. The chain culminates in latitude 52°, 

 where the mountains are very massive and are traversed by profound 

 valleys, the heads of which are occupied by glaciers. 



From the Rocky Mountains to the Pacific Ocean the country is 

 extremely rugged, resembling similar Silurian and metamorphic 

 regions in other parts of the world. It forms a great trough, 

 bounded to the west by the Cascade range of mountains, which 

 closely hugs the Pacific coast in this latitude. This range, which 

 is only rarely broken by valleys, and those of comparatively recent 

 date, runs like a wall 4000 to 5000 feet above the sea-level. At 

 intervals there occur great conical mountains, such as Mount Hood, 

 Mount Baker, and others, which rise to 10,000 or 12,000 feet, and 

 from their isolation, being perfectly unconnected, except by the 

 lower range, they present a very grand appearance when viewed 

 from the coast. Owing to the great fall of the rivers, the narrow 

 valleys, and the rapid erosion having continually carried on the re- 

 arrangement of the superficial deposits, the grades in the elevation 

 of the continent cannot be so well discerned on the western slope as on 

 the eastern, although these deposits are found to be greatly developed. 



After this brief sketch of the physical features of the country, I 

 now proceed to describe the different strata, reversing the order of 

 their deposition. 



STTPEEEICLA L DEPOSITS. 



These are very extensively developed in every part of the region 

 explored; and their classification involves veiy interesting conclusions 

 respecting the changes of level of the continent, both posterior and 

 anterior to the great northern drift. 



