430 STEVENSON— THE FORMATION OF COAL BEDS. i^ov. i, 



of the Appalachian revoUition brought deposition to an end in the 

 comparatively small area which remained. 



Before presenting in detail the evidence on which these state- 

 ments are based, it is well to call attention to a matter respecting 

 which some misapprehension seems to exist. Time breaks in 

 deposition, due to existence of a land surface, do not leave in every 

 case a record in the way of non-conformity, which can be recog- 

 nized in even a considerable area. Schucherf* has emphasized in 

 this connection the conditions observed in the neighborhood of 

 Louisville, Kentucky, where the limestone deposit is conformable 

 throughout and appears to be continuous. The Devonian portion 

 can be distinguished from the Silurian only by the fossils, although 

 the portions are separated by a long land-interval. He offers many 

 other instances to which the writer may add one, already referred 

 to. In Westmoreland and Fayette counties of Pennsylvania, the 

 Upper Pocono or Logan is shown resting conformably upon the 

 Chemung, while at 60 miles east those formations are separated 

 by not less than 4,000 feet of Catskill and Lower Pocono. Nor is 

 it in any sense necessary that there be extensive erosion during a 

 somewhat prolonged period of sub-aerial exposure, if the land be 

 level or low-lying, even though the period be long enough to admit 

 of the cutting of considerable valleys. Schuchert has shown that, 

 though exposed during the whole period since the Taconian revolu- 

 tion, the Cincinnatian region has lost certainly little more than 400 

 feet by erosion and that in all probability the greater part of this 

 loss has occurred since the Pleistocene elevation. Ulrich'' says 

 that a limestone in the St. Louis area, 5 feet thick, was exposed 

 during the Silurian and the Devonian, yet it was not removed. 

 Illustrations of the slowness of erosion, where land is level, are 

 abundant in portions of Vermont, where one finds broad, terraced 

 valleys in the Quaternary sands and gravels. The region has always 

 been one with heavy rains, yet in some extended spaces the broad 

 upper terrace is only slightly scarred, though, since it was aban- 



' C. Schuchert, "Palaeography of North America," pp. 441, 442. 

 * E. O. Ulrich, " Revision of the Palaeozoic Systems," p. 306. 



