86 Traxsactions of the American Institute. 



We are not to infer, however, that because we have seen tliese^ 

 evidences of the oscillation of the ocean bed so conspicuously marked 

 during the Coal period, that similar conditions did not exist in pre- 

 ceding geological periods. In this period we have the changes more 

 strongl}^ marked by the presence of land plants in strata which alter- 

 nate with beds containing marine fossils ; while in preceding geologi- 

 cal periods, the change is mainly marked by the alternations of lime- 

 stone beds of the quiet ocean with strata of sedimentary origin, both 

 containing marine organisms. In each one, however, the conditions are 

 indicative of oscillation or changes of level in the ocean bed ; evi- 

 dences of that earlier movement, which has finally given us the 

 North American continent in its seco"nd phase. Glancing over the 

 map which is before us, you will see that all this portion of the continent 

 is occupied by rocks of those geological ages of which I have been 

 speaking. It is true, that along the coast, and in a few of the 

 valleys, jou have geological formations of later date, which have 

 been elevated above the ocean level by subsequent movements of the 

 continent, but they form no essential part of the continental features 

 of that period. 



It is not a little interesting to observe that the source and distribu- 

 tion of the materials of all these sedimentary deposits is essentially 

 the same ; each successive period spreading its coarser sediments in 

 the same direction, or with a gradually more western trend in the 

 later deposits, and finally, and as a consequence of this, the earliest 

 dry land appearing in the direction of this source of the sediments. 



Having passed in review these successive formations, we naturally 

 still inquire how they have become dry land, and not only dry 

 land, but a considerable portion of this area of mountain elevation. 



It is the common belief that naiountain chains are due to some 

 violent action, as the breaking and uplifting of the crust along certain 

 lines ; and this has been accepted as an explanation of the elevation 

 of mountains and adjacent tracts of country. Now, however, since 

 we have more carefully studied the structure of mountain chains, and 

 the nature of their materials, we find them composed of the same 

 kinds of sediment as the rocky strata of other parts of the country. 

 We find, in short, that they are composed of stratified rocks ; and 

 however much they have become changed in position or in their 

 condition, they are the results of sediments deposited in the ocean. 

 Although in certain parts they are highly crj'stalline, we yet some- 

 times detect organic remains, and the lines of regular bedding still 



