ATLANTIC COAST BASINS. 39 



sumption that this trough is old, and that in the Carboniferous period, if it 

 had been sufficiently low lying, it would have afforded a favorable field 

 for the accumulation of strata, is supported by the fact that Helderberg 

 strata are found within its bounds, and the absence of the later Paleozoic 

 is fair proof that the trough was so placed that it remained subjected to 

 erosion. In this connection it is interesting to note that the Triassic 

 period does not appear to have introduced true marine conditions along 

 this coast line. The fossils indicate that the beds were formed either m 

 fresh-water lakes or in estuaries. So, too, the Carboniferous strata of the 

 East Appalachian district contain only fresh-water fossils, notably lacking 

 the thin beds containing marine fossils which in the West Appalachian 

 district clearly indicate successive invasions of the sea. These facts are 

 best explicable on the supposition that the Atlantic coast in this part of its 

 history lay farther to the east than it does at present, and that all the beds 

 from the beginning of the Carboniferous upward through the greater por- 

 tion of the Triassic section were formed in basins so far separated from the 

 sea that no marine life found access to them. 



The development of fresh-water basins on the Atlantic coast in the Car- 

 boniferous period has perhaps its parallel on a larger scale in that curious 

 formation of shallow lakes which occurred in Cretaceous time along the 

 eastern border of the Cordilleras of North America, and which gave during 

 the Mesozoic and a part of Tertiary time the nearly continuous fresh-water 

 areas from Texas to the high North. Depressions of this nature appear to 

 be of common occurrence along the bases of mountain ranges which have 

 recently been subjected to extensive movements. It seems possible, indeed, 

 that they are due to couuterthrust action, which tends to bear down the 

 part of the earth immediately outside of the field of considerable elevation. 

 Phenomena of this sort are traceable not only in this country, but around 

 the margin of the Alps and other mountain districts which have been suffi- 

 cientlv well mapped to give indications of these old basins. On this suppo- 

 sition we can account for the general tendency of the East Appalachian 

 district to subside during the time when the neighboring ranges were under- 

 going elevation. The intensification of this subsidence at particular points 

 and the consequent infolding of strata, which have thus been preserved from 

 erosion, is to be explained through the accumulation of thick deposits of 

 unconsolidated rocks in preexisting erosion troughs. 



