28: [Assembly 



tents. Such a series of all the rocks older in date than the 

 great Coal formation exists in New- York, much better exhibited 

 than in any other region yet known. 



The same series exists in great thickness along the range of 

 the Alleghanies ; but they are in that region so much upheaved, 

 displaced and contorted that it is very difficult to trace their 

 edges so as to learn their true position and succession. In that 

 range, also, fossils are few; either from their original scarcity in 

 that part of the ancient sea, or because subsequent changes in 

 the rock have obliterated their remains, so that the all-important 

 aid which they afford in studying the strata can be but sparingly 

 obtained. 



In New-England it is still worse ; for there the rocks are not 

 only broken up and distorted in position, but so changed^ or 

 metamorphosed by the action of heat and other causes, that all 

 fossils are obliterated except in a few rare instances, and the 

 rocks are recognizable in their true character with great diffi- 

 culty. 



In the Western States the case is different : there these strata 

 are almost as level as when originally formed, and they contain 

 many well preserved fossils. But in that direction many of them 

 " thin out," or diminish in thickness ; so that while some impor- 

 tant groups of strata quite disappear before reaching Ohio, 

 others, which in New York are many hundred feet thick, are in 

 the West but a few yards ;* and the level and unbroken character 

 of most of that region, covered with gravels or alluvial soil, pre- 

 vents the rocks from being well examined in many districts. 



* The thinning out of the rocks westward is very remarkable. Tracing them from New- 

 York and Pennsylvania to Iowa, we find the Hudson river group and Oneida conglomerate 

 diminish from 2000 to 100 feet; the Medina sandstone runs out entirely; the Onondaga-salt 

 group diminishes from 1000 to 160 feet; the Hamilton, Portage and Chemung groups, from 

 4000 to 200 feet; the Catskill group, from 3000 feet in New-York, disappears entirely in the 

 West; and the Carboniferous, 6000 feet thick in Pennsylvania, is only 600 in Iowa. And it 

 is also true that all, or nearly all, the coarser sandy rocks, traced westward from New-York, 

 are found to become finer-grained and more calcareous, as well as thinner; while the lime- 

 stones generally keep their thickness from east to west, with little or no diminution. 



These facts lead to the belief that the source whence the materials of these rocks were 

 derived was at the East; and that while the old marine currents spread the sediments very 

 freely and heavily in this region, only a small portion of them, and that of the finest and 

 lightest kind, was carried into the •western part of the old ocean to form the thin and fine- 

 grained strata of Iowa, Wisconsin and Minnesota. 



The same strata above mentioned as so thick in New-York, are far thicker still in the range 

 of the Alleghanies, and also in the continuation of their range through the Green mountains 

 and the White mountains (where they have been much changed, as will be hereafter shown) ; 



