112 GEOLOGY. 



and how far the result of the removal of the later beds by erosion. 

 The unconformity between this series and the (Upper) Cretaceous 

 above shows that erosion removed some of the former, before the 

 deposition of the latter. 



Constitution and structure of the Potomac and Tuscaloosa series. — 

 In its mode of formation the Potomac series appears to belong to the 

 less familiar of the two great classes of deposits, the terrestrial, as 

 distinguished from the marine. As already noted, the whole eastern 

 mountain and plateau region seems to have suffered peneplanation 

 during the Jurassic period, attended inevitably by the deep decay 

 of the underlying crystalline and other rocks, and the consequent 

 accumulation of a heavy mantle of residuary earth and insoluble rock. 

 The warping which inaugurated the Comanchean period seems to have 

 involved a rise of the axis of the Appalachian tract, and a consequent 

 rejuvenation of the drainage from it, while the coastward tract was 

 left relatively flat, or perhaps bowed into a concave attitude, making 

 it a zone of lodgment for the sediments brought down from the west. 

 The quickened drainage of the axial tract, acting on material prepared 

 for easy removal, loaded itself with a burden it could not carry across 

 the low coastal tract, and deposition resulted. It is perhaps not 

 necessary to assign concavity or permanent submergence to the lodg- 

 ment tract, if the loading of the rejuvenated head- waters of the 

 streams was sufficient; but lakes, marshes, etc., were probably features 

 of the area. These conditions are in harmony with the constitution 

 of the deposits, which consist of gravel (or conglomerate), sand (or 

 sandstone), and clay. 



The gravel (or conglomerate) at any point is made up principally 

 of materials derived from the formations adjacent on the west, and 

 subordinately from the subjacent formations. It is often arkose in 

 the immediate vicinity of the feldspar-bearing crystalline rocks, but 

 elsewhere it is composed chiefly of the resistant products of mature 

 weathering. Among these, quartz, from the quartz veins of the crys- 

 talline rocks is often conspicuous. Chert, quartzite, and sandstone from 

 the Appalachians, are also constituents. The gravels are sometimes 

 disposed irregularly, constituting lenses or beds of varying thickness. 



The sands are sometimes fine and the grains .well rounded, as if 

 long transported by moving water, and sometimes coarse and angular, 

 as if they had been subjected to but little wear. Like the gravel, 



