^a 7'l?AXSACTI0XS OF THE AmERICAX IxSTITUTE. 



our continent, as a whole, "was not formed or called into existence at 

 once, nor was it elevated from beneath the ocean as a single continent 

 in its present form and extent. It has been produced step bj step, 

 and by slow accumulation through many long geological periods. 

 The materials of which it is composed have been derived from the 

 breaking down of pre-existing continents and islands, and the trans- 

 port and deposit of the sediments thus formed, until one portion after 

 another has invaded the oceanic area and become dry land. Each 

 portion thus rising above the ocean level has furnished, through the 

 action of the elements as I have shown you, materials to be distributed 

 over the ocean-bed for the foundation of the next succeeding dry land, 

 and thus by a process of destruction and reconstruction, constantly 

 going on, the ecpiilibrium of the forces is kept up ; the existing state 

 of things at any epoch being only a phase preparatorj'- to a subse- 

 quent change. 



The greater changes which have taken place are obvious to all, but 

 the intermediate steps or progress are not so readily seen. We speak 

 of the geological formations and their succession ; but within all these 

 are minor and less conspicuous changes. We see among the formations 

 termed Silurian and Devonian, some twenty or thirty different epochs, 

 each marked by its characteristic fossils of fauna and flora, iiidicating 

 as many minor changes in the condition of the ocean bed. In the sedi- 

 ments of each of these phases, we have probably as many species of 

 marine animals as are now living along the coast of the United States. 

 When we consider that these various animals have lived and died, that 

 each has occupied its place for successive generations and for an 

 unknown length of time ; wlien we consider that this area has been 

 covered entirely by subsequent deposits, and other creations have 

 taken their place, and so onward, while accumulations hundreds of feet 

 thick have been spread over them ; when we remember that hundreds, 

 and even thousaTids of these generations have lived and died, perhaps 

 in each of those twenty or thirty subdivisions of the period, and thus 

 onward, fauna after fauna, and flora nfter flora, through all these 

 epochs, you have at last an incomprehensible number of generations 

 of animals, a result which could only have been reached by a process 

 carried on for an indefinitely long period of time. One point which I 

 have endeavored to impress upon you is, that wliile this has been 

 going on, there has been, so far as our own continent is concerned, 

 a constant evolution of dry land. If we begin at the latest period, 

 and go backward through those preceding it, you have in them all the 



