REPORT ON THE GEOLOGY OF NORTH AMERICA. 5 



cut into the valleys and troughs which it presents by the retreat 

 of the upheaved waters. The submarine origin of all this tract 

 will be made apparent in treating of its geology ; but in refer- 

 ence to its valleys, it may be well to remark that it has no doubt 

 been torn by more than one denuding wave, in as much as the 

 great current which has evidently rushed over other portions of 

 the continent has also passed across this tract, and strewed it as 

 we see with diluvium. How many such denudations of the 

 strata have operated to form the present broad valleys of its 

 enormous rivers, or how much of the excavation has been due 

 to the continued action of the rivers themselves, we have, so far 

 at least, no sufficient data to form a decision. 



The level region here spoken of I propose calling, for conve- 

 nience, the Atlantic Plain of the United States, while the ter- 

 ritory between it and the mountains may be fitly entitled the 

 Atlantic Slope. 



The extensive denudation of the surface of this plain will be 

 found highly favourable to the accurate development of its geo- 

 logy. It is from this and the accessible nature of its rivers that 

 we already know more of its strata, and especially of its organic 

 remains, than we do of any other district of the country. Its 

 horizontal strata are in many places admirably exposed in the 

 vertical banks of the rivers, often through many miles' extent; 

 and the mass of appropriate fossils thus procured, as will be 

 seen from this Report, is already far from insignificant. This 

 plain, widening in its range to the south-west, bends round the 

 southern termination of the Alleghanies in Alabama, and expands 

 itself into the great central plain or valley of the Mississippi. 

 The tract in question embraces the greater portion of the newer 

 secondary and tertiary formations hitherto investigated upon 

 this continent, though, notwithstanding the great area it covers 

 from Long Island to Florida, it may yet be found to constitute 

 but a small section of the whole range of those deposits, when 

 we shall, on some future day, have explored in detail the vast 

 plains beyond the Mississippi. 



The ledge of primary rocks, bounding the tertiary and cre- 

 taceous secondary deposits of the Atlantic coast, may be de- 

 lineated by commencing at the city of New York, and tracing a 

 line marked out by the falls in nearly all the rivers from thai 

 point to the Mississippi. It is thus marked in the falls of the 

 Passaic at Patterson, in the Raritan near New Brunswick, in 

 the Millstone near Princeton, in the Delaware at Trenton, the 

 Schuylkill near Philadelphia, the Brandywine near Wilmington, 

 the Patapsco near Baltimore, the Potomac at Georgetown, the 

 Rappahanock near Fredericksburg, James River at Richmond, 



