350 A. KEITH OUTLINES OF APPALACHIAX STRUCTURE 



rian. wherein the deep basin filled with Cambrian and Ordovieian sedi- 

 ments was raised above the sea and there remained nntil the latter part 

 of the Devonian. While erosion was going on at the south deposits were 

 still formed in the basin at the north. A similar event took place, but in 

 the opposite direction, when the sediments of the Mississippian were ex- 

 posed by the rising sea-floor at the north and worn away, only to he again 

 depressed and receive another load of sediments. The maxima of uplift 

 are opposite in the two cases. 



The bearing of these movements is this : While the sedimentary basin 

 was actually above land, what started it again on its downward course? 

 Surely not any fresh deposition of sediment, because there was no such 

 deposition until alter the depression began. Further, while it was be- 

 neath the sea and being loaded more, why should it rise in spite of the 

 added load ? During the Pennsylvanian the deposits of the coal measures 

 were laid down in a basin deej^ening with great steadiness and slowness, 

 as evidenced by the successive beds of coal and their land flora. Why, 

 indeed, "should the land have been depressed at all, if isostasy was the 

 cause of uplift of the land? How could deposition have started unless 

 some force had provided a basin? 



The most striking case of all was at the beginning of the Upper Cam- 

 brian. For untold ages the Precambrian granites and gneisses of half 

 the continent had 1)een eroded from the land and their waste carried 

 eastward into the sea. Then, half of this region sank beneath the sea 

 and a new land, Aj^palachia, rose from the waves of the east. If isostasy 

 ruled, why did the land and sea reverse their positions ? What depressed 

 the land when it had suff'ered a maximum of erosion, and why did the 

 sea-floor rise in spite of an ever-increasing load? Obviously, not on ac- 

 count of isostasy but in spite of it, for the results are the opposite of 

 those of isostasy. It remains for us to conclude that another cause more 

 potent than isostasy lay at the root of these movements and that its 

 major efl'ects were at times hindered and at times helped by the action 

 of isostasy. 



To sum up. there are eight chief obstacles to acceptance of the theory 

 tliat isostasy caused the folded mountain range: First, isostasy is not in 

 itself an initiating cause, but requires ^ome other force to start its oper- 

 ation. Second, isostasy is a continuing j^rocess : therefore it requires 

 some outside force to sto]) its action ; such stoppage is known to have 

 occurred repeatedly. Third, the factors which are appealed to as main- 

 taining isostasy are differences of density and gravity so slight that they 

 can be readily overpowered l)y other causes: that they were so over- 

 powered is clear from the repeated and vigorous reversals of isostasy 



