500 REPORT OF NATIONAL MUSEUM, 1904. 



formulated his views and gave them in extension in part six of the 

 third volume of the Natural History of New York. In 1869, in an 

 address on the Geological History of the North American Continent, 

 delivered before the American Institute of New York, he reiterated 

 many of the opinions previously announced, and, finally,, in 1882 

 brought out the original address of 1857, and this, it is said, without 

 revision. Whatever changes or additions it was found desirable to 

 make were added in the form of supplementary notes. The subject 

 may. therefore, be conveniently reviewed at this date (1859). 



Hall had shown that one simple and intelligible sequence of strata, 

 from the Potsdam sandstone to the end of the Coal Measures, covered, 

 with slight exceptions, the entire country from the Atlantic slopes, to 

 the base of the Rocky Mountains, and that while the horizontal strata 

 gave their whole elevation to the highest parts of the plain, the same 

 beds were folded and contorted in the mountain region, thus giving to 

 the mountain elevation not one-sixth of their actual thickness. He 

 thought to have shown conclusively that the line of greatest accumu- 

 lation of sediments had been along the direction of the Appalachian 

 chain; in other words, that the Appalachian chain was itself due to 

 the original deposition of materials and not to any subsequent action 

 or influence breaking up and dislocating the strata of which it was 

 composed. 



Discussing the cause of this folding and plication, he referred to the 

 tact previously recognized by Herschel to the effect that sea bottoms, 

 when loaded by accumulated sediments, undergo a process of subsi- 

 dence which may cause an elevation of the adjacent continental areas. 

 a principle which was then becoming generalh" recognized and which 

 lias since become known under the name of isostacy. When, then, these 

 sediments were spread along a belt of sea bottom, as originally in the 

 line of the present Appalachian chain, the first effect would be to pro- 

 duce a yielding of the earth's crust beneath and a gradual subsidence. 

 Evidence of this subsidence was furnished by the great amount of 

 material accumulated, for it was impossible, he argued, to suppose 

 that the sea had been originally as deep as the thickness of the accu- 

 mulations (some 40,000 feet). 



The line of greatest depression would, therefore, be along the lines 

 of greatest accumulation. By such a process of subsidence the lower 

 side of the accumulations would become gradually curved and stretched, 

 and there would follow, as a sequence, rents and fractures. On the 

 surface above, which would be contracted horizontally by such sub- 

 sidence, there would be produced wrinkles and foldings of the strata. 

 Into the rifts formed below it was conceivable there might rush fluid 

 or semifluid material, producing what are now evident as trap dikes. 



The sinking of the mass would produce a great synclinal axis, and 

 within this axis, whether on a large or small scale, would be produced 



