50 THE ORIGIN OF MOUNTAINS. [V. 



coal-measures come to be of little importance, and we have a 

 great limestone formation of marine origin, which in the Rocky 

 Mountains and Xew Mexico occupies the horizon of the coal, 

 and, itself unaltered, rests on crystalline strata like those of the 

 Appalachian range. In truth, Mr. Hall observes, the carbon- 

 iferous limestone is one of the most extensive marine formations 

 of the continent, and is characterized over a much greater area 

 by its marine fauna than by its terrestrial vegetation. 



"The accumulations of the coal-period were the last that 

 gave form and contour to the eastern side of our continent, from 

 the Gulf of St. Lawrence to the Gulf of Mexico ; and as we 

 have shown that the great sedimentary deposits of successive 

 periods have followed essentially the same course, parallel to 

 the mountain ranges, we naturally inquire : What influence 

 this accumulation has had upon the topography of our country, 

 and whether the present line of mountain-elevation from north- 

 east to southwest is in any way connected with the original 

 accumulation of sediments." (Hall's Paleontology, Vol. III. ; 

 Introduction, p. 66.) 



The total thickness of the palaeozoic strata along the Appala- 

 chain chain is about 40,000 feet, while the same formations in 

 the Mississippi Valley, including the carboniferous limestone, 

 which is wanting in the east, have, according to Mr. Hall, a 

 thickness of scarcely 4,000 feet. In many places in this vallcy 

 we find the paleozoic formations exposed, exhibiting hills of 

 1,000 feet, made up of horizontal strata, with the Pots.l.uu 

 sandstone for their base, and capped by the Xiagara limestone ; 

 while the same strata in the Appalachians would give from ten 

 to sixteen times that thickness. Still, as Mr. Hall remarks, we 

 have there no mountains of corresponding altitude, that is to 

 say, none whose height, like those of the Mississippi valley, 

 equals the actual vertical thickness of the strata. In tin 

 there has been little or no disturbance, and the highest eleva- 

 tions mark essentially the aggregate thickness of the strata com- 

 posing them. In the disturbed regions of the east, on the con- 

 tlmu^h we can prove that certain formations of kn-iwn 

 thickness are included in the mountains, the height of these is 



