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Mesozoic aad Cenozoic Geology and Paleontology. 183 
MESOZOIC AND C4INOZOIC GHOLOGY CONTINU ED— 
THE DRIFT OF THE CENTRAL PART OF THE CON- 
TINEN T. 
By 8. A. Mitier, Esq. 
| Continued from Vol. iv., page 144. | 
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 furrows upon the rocks; the ancient soil beneath the drift ; and 
the animal and vegetable remains which immediately preceded the 
drift, and also such as are found within it. 
It is idle to talk of continental elevations or depressions, for the 
whole science of geology and paleontology 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 already 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 slowly 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 paleeozoic 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 
