Scientific Lectures. 89 



expand the mass, and produce an upward movement of the Avhole. 

 The equilibrium of temperature and of pressure will be gradually 

 restored, and the elevation of the area will in some degree corres- 

 pond with the thickness of the original accumulation ; but all parts of 

 any zone, over which sediments have been deposited in a certain 

 period, will be aifected by the movement. This slow elevating pro- 

 cess will not bring back the relative level of the old sea bottom with 

 that of the present ocean, simply in consequence of the unequal disr 

 tribution of the newly added sediments ; and although the height of 

 the mountains will depend upon the thickness of the strata, you can 

 never have them of a height above the sea equal to the. entire thick- 

 ness of the beds of which they are composed. f) 



I have just now shown you that this great area from the Atlantic 

 to the Mississippi river, and from Nova Scotia to Georgia, consists of 

 a series of strata of the same age, but that the nature of the sedi- 

 ments, and the degree of accumulation is unequal in different parts 

 of the area. While we have along the eastern zone, some 30,000 or 

 40,000 feet of thickness, there is only about a tenth part as much in 

 the west. The elevation of the plateau of the Mississippi Yalley 

 region above tide water, may be something more than one-fourth of 

 the entire thickness of the underlying palaeozoic strata, while the 

 elevation of the highest mountains of the Apalachian chain is not 

 quite one-fourth the thickness of the strata of the same age along 

 that zone. IS^ow, you will readily conclude from this fact, that 

 there remains no reason for calling in the aid of extraordinary 

 forces for the production of these mountains. The plateau of th« 

 west is proportionally as much elevated as are the highest points of 

 the eastern range. It requires only to be convinced that you 

 are dealing with the same set of rocks throughout this extent 

 of country, and you cannot avoid the conclusions which I 

 have presented. The gentler elevations or the abrupt terraces and 

 deep ravines in the comparatively level western country are quite as 

 great in proportion to the thickness of the strata as are the mountain 

 elevations, steep escarpments, and deep gorges of our eastern mountain 

 region. All the changes which have taken place from the first remo- 

 val and transportation of the sediments to the production of the pre- 

 sent continent, in all the features which it carries upon its surface, 

 are due to the operation of the few simple laws which I have 

 enunciated. 



I have dwelt longer upon this part of our continent than it 



