Stevenson.] Zv4: [Dec, 18, 



land was continually becoming fitted for its support. During repose, 

 deltas were formed in the bay, and the marsh pushed outward on the 

 newly -formed land. If the period of repose were long enough to permit 

 the bay to be filled up, the marsh would cross to the other side if begun 

 on only one, or, if pushing out from all sides, it would reach the centre. 

 The Pittsburg, Redstone, Sewickly and Waynesburg originated at the 

 east, for there they attain their greatest thickness, while westward they 

 diminish. VIIIc, IX and X of the Ohio section are thickest westward 

 and then eastward, the first barely ci'ossing the Ohio river ; the others 

 disappearing before they reach it. 



It may be objected that a marsh i-equires an almost level plain for i*s 

 existence. Nothing could be more erroneous than such a supposition, 

 for all necessary conditions may exist on a hill-side with not too steep a 

 slope. In Colorado, I found on Arkansas Pass, near the head of the 

 Arkansas river, an immense morass covering the whole surface between the 

 canon walls, a distance of more than one-fourth of a mile. It reaches for 

 several miles down the canon, whose floor has a fall of nearly two degrees. 

 This is no petty swamp. To all intents and purposes it is a bottomless 

 morass, almost impassable to mounted stock. 



There is every reason to suppose that previous to the upper coal epoch, 

 the conditions were by no means so regular throughout the basin. It is 

 highly probable that just before the beginning of that epoch, the trough 

 was narrowed and the eastern border, at least, much raised. Otherwise 

 it would be difficult to explain why it is that the Pittsburg Coal does not 

 distinctly overlap the lower Barren group. At times during the lower 

 coal epoch the folding process must have been carried on quite energeti- 

 cally, much more so than during the epoch of the upper coals . In the 

 latter there are found no subordinate folds such as are exhibited in the 

 former ; such, for example, as occurred previous to the formation of the 

 Kittanning so as to i^roduce the secondary troughs in which that coal lies 

 causing so great variations in the thickness of the interval between it and 

 the Upper Freeport. It seems quite possible, judging from some obser- 

 vations in Ohio, that similar subordinate foldings may have taken place 

 previous to the formation of Coal III, the next below the Kittanning. 



In view of the facts given in this paper, I feel justified in extending my 

 statement that the Indiana arid Appalachian coal-fields were not con- 

 nected during the lower barren and upper coal epochs, by asserting that 

 there is no reason to suppose that they were ever united north from Ken- 

 tucky, Whether or not they were united farther toward the south must 

 be determined by others. 



Thus far no reference has been made to the trough or basin lying east 

 from the Alleghany Mountains and holding the Barren and the Upper 

 Coal Group. The terrific erosion which this region has suftered, only 

 fragmentary areas of coal remaining, renders the collection of details a 

 work of great difficulty, and few observations exist, which bear upon the 

 question under discussion. This basin and the Great Bituminous Trough 



