Scientific Lectures. 83 



vcally that the material was derived from the east and north-east, 

 because the coarser part is deposited along this line, and the finer is 

 carried towards the center of the continent ; for it is evident that the 

 finer sediments would be transported by the gentler currents, while the 

 coarser ones would not. Throughout the "West, between the 

 Apalachian chain and the Mississippi river, you have a great 

 plateau, covered almost altogether with fine sediments or calca- 

 reous mud. It is not because this is of a difterent age from the 

 mountain range ; it is not because of any change that it has 

 undergone, but it is simply that along the line of the Apalachians 

 the coarse sediments were deposited by the ancient currents, thus 

 formiug tlie outline of the continent. This outline is due to no 

 other cause whatever than the deposition of these coarser materials 

 along the course of the ancient oceanic currents ; while the finer 

 were swept, by gentler movements, farther towards the center of the 

 gi'eat ocean. 



Before we proceed further, let me refer to this chart which represents 

 the difierent strata as we should find them passing southward from 

 the region of the Adirondack mountains. You will find the different 

 formations represented here, one above another, this being a section, 

 cutting down through the entire series — the Silurian lower and 

 upper, the Devonian, and the Carboniferous. Those of which I 

 have been speaking especially are the Silurian ; but we have, "never- 

 theless, complicated with them in the mountain regions, rocks of 

 Devonian and Carboniferous age. As soon as we pass to the 

 westward from the disturbed region, these rocks become highly 

 fossiliferous ; and I have in my hands a mass of stone, of Devonian 

 age, taken from the eastern part of ISTew York, which is crowded with 

 shells as closely as you can find them now imbedded in the mud 

 upon the sea coast. Though we have strata of a thickness of 

 thousands of feet characterized by similar shells, and marine plants 

 of littoral growth, yet we know this was deposited in proximity to 

 the shore, or in shallow water, from the character of those fossils, 

 which belong to the class of animals living; near the shore. Such 

 accumulations of sediment, of many hundreds of feet in thickness, 

 everywhere giving evidence of shallow water, could only have been 

 made upon a sea bottom which was being gradually depressed. 



As you pass away from this shore line, these forms gradually diminish 

 in numbers until you have none of the same shells whatever ; but you 

 get at last, in the calcareous mud throughout the great plateau of the 



