Lesley.] ^gg [December. 



surface of the Cretaceous age, by the sweeping and garnishing artists 

 of Tertiary times. Little by little the whole sloping mean horizon of 

 water-level, from the Alleghany Mountain to the South Mountain, was 

 lowered to its present line. The gaps were gradually deepened, 

 widened, and rounded off to correspond with the slow deepening of 

 the limestone and slate valleys behind them ; and the long strait 

 narrow sandstone crests of the mountains of IV and X and XII 

 (Middle Silurian, Upper Devonian, and Carboniferous), were gnawed 

 away evenly at a slower but not less steady rate. 



It was Professor Rogers's opinion that all this was, so to speak, the 

 work of a moment ; the consequence of the rush of a large body of 

 water over the face of the Continent, at the time when the coal era 

 was abruptly brought to a termination by the upheaval into the air 

 of the whole Appalachian belt of earth-crust, when it was thrown 

 into waves or folds; after which the once horizontal strata remained 

 partly or entirely upright. 



With this cataclysmic hypothesis I cordially sympathized for 

 some years ; and some of the geologists of the Pennsylvania survey, I 

 believe, still do so. Nor am I yet entirely convinced, — it may be 

 from the force of a strong and early prejudice, — that such a cataclysm 

 is not indispensable to explain the earlier and perhaps the larger 

 part of the whole phenomenon. Not that I ever accepted that part 

 of Mr. Rogers's statement of it which gave an account of the modus 

 operandi of the anticlinals, viz. : by a pulsating planetary lava-nucleus. 

 But the study of the surface itself, covered with mountains and val- 

 leys, arranged in a beautifully symmetrical manner, by whatever ener- 

 gy you please, — and I have always thought the lateral thrust of a 

 cooling and shrinking crust one sufficiently plain and precisely ex- 

 planatory of the details, — in fact, the study of these details, some of 

 which offer the most inviting problems of erosion to the structural 

 geologist, has impressed upon my mind the conviction that aerial and 

 fluvial agents are not the kind which could have begun the great 

 work of Appalachian erosion. Give them time and they are omni- 

 potent, I grant, hut only in their own sphere. 



It would lead too far to argue this part of the subject here. I only 

 wish, when I describe the whole water-shed horizon of the Appala- 

 chians as being step by step lowered during later Secondary, Tertiary, 

 and Quaternary times, to guard against that total rejection of cata- 

 clysmic agency which has come to characterize the geological specu- 

 lation of the present day upon great structural questions. This fact, 

 evidently true in itself, is also necessary to the argument respecting 



