Mesozoic and Coenozoic Geology and Palmontology. 287 



We now come to the consideration of the sand, gravel and bowlders 

 constituting the drift of the central part of the continent; the scratches 

 and farrows upon the rocks; the ancient soil beneath the drift ; and 

 the animal and vegetable remains which immediateh'' preceded the 

 drift, and also such as are found within it. 



It is idle to talk of continental elevations or depressions, for th| 

 whole science of geology and palaeontology teaches us of the gradual 

 growth or formation of continents. The appearance of islands above 

 water, until an archipelago is formed, followed by the slow filling up of 

 the shallow places and the intermittent local elevation of mountain 

 chains, through vast geological ages, until the islands are thoroughly 

 united into one vast body or continent, is the history of all continental 

 elevations, and science teaches us of none other, and if continents have 

 been depressed they must now be beneath the ocean, for we know 

 nothing of such phenomena. 



We have alreadj' seen the vast deposits of the Triassic and Jurassic 

 periods, followed by the marine and brackish water deposits of the 

 Cretaceous age that so well nigh formed the outlines of this continent. 

 The elevation of the mountain chains that caused the formation of 

 vast internal lakes, which have slowl}^ drained themselves through all 

 Tertiary time, and the slight elevation of some parts of the coast during 

 the same period has given us the present form of our continent. 



As soon as an island appeared above the ocean the denudation of 

 its surface, from atmospheric causes, began. The rains at once com- 

 menced the excavation of valleys and ravines, and when the islands 

 began to assume a continental shape, the valleys must necessarily 

 have been correspondingly increased in size. As the Appalachian 

 range dates back, in part, as far as the close of palaeozoic time, so 

 the Ohio river and other streams from this mountain chain have the 

 same age. Another drainage system existed from the Laurentian 

 mountains by a way that has been interrupted and thrown into a 

 series of lakes, but the ancient valley has been traced from Lake 

 Huron through Lakes Erie and Ontario. To the west and north of 



