On American Geological History* 427 



The Taconic si/stem of Emmons has been supposed by its au- 

 thor to have a place inferior to tlie Cambrian of Sedgwick, or 

 else on a level with it. But the investigations of Hall, Mather 

 and Rogers, and more lately of Logan and Hunt, have shown 

 that the Taconic slates belong with the upper part of the Lower 

 Silurian, being, in fact, the Hudson River shales, far from the 

 bottom of the scale. 



HI. The American rocks throw much light on the origin of 

 coal. Professor Henry D. Rogers, in an able paper on the 

 American coal-fields, has well shown that the condition of a delta 

 or estuary for the growth of the coal-plants, admitted even now 

 by some eminent geologists, is out of the question, unless the whole- 

 continent may be so called ; for a large part of its surface was 

 covered with the vegetation. Deltas exist where there are large 

 rivers ; and such rivers accumulate and flow where there are 

 mountains. How, then, could there have been rivers, or true del- 

 tas of much size, in the Coal Period, before the Rocky Moun- 

 tains or Appalachians were raised? It takes the Andes to make 

 an Amazon. This remark has a wider application than simply 

 to the Coal Era. 



IV. In this connection, I add a word on the idea that the rocks 

 of our continent have been supplied with sands and gravel from 

 a continent now sunk in the ocean. No facts prove that such a 

 continent has ever existed, and the whole system of progress, as 

 I have explained, is opposed to it. Moreover, gravel and sands 

 are never drifted away from sea-shores, except by the very largest 

 of rivers, like the Amazon ; and with these, only part of thehght- 

 est or finest detritus is carried far away ; for much the larger 

 part is returned to the coast through tidal action, which has a 

 propelling movement shoreward, where there are soundings. The 

 existence of an Amazon on any such Atlantic continent in Si- 

 lurian, Devonian, or Carboniferous times, is too wild an hypothe- 

 sis for a moment's indulgence. 



V. The bearings of the facts in American Pah-eontology on the 

 science, might well occupy another full discourse. I will close 

 with brief allusions to some points of general interest. 



1. The change in the Fauna of the globe as the Age of Man 

 approaches, is one of the n^.ost interesting facts in the earth's his- 

 tory. It w^as a change not in the types of the races, (for each 

 continent retains its characteristic,) but a remarkable dwindling 

 in the size of species. In North America the Buflalo became 



