440 ORIGIN OF THE APPALACHIAN COAL STRATA, 



about thirty miles to the northwest, the thickness of the formation 

 is only about five hundred feet ; while still further across the chain, 

 where it becomes the general floor of the coal measures under the 

 bituminous form, in the basins northwest of the Allegheny moun- 

 tain, its entire thickness seldom exceeds eighty or one hundred 

 feet. Tracing it across the great western coal-field, until we en- 

 counter its last outcrop in western Pennsylvania, Ohio, and Ken- 

 tucky, this wonderfully expanded rock, dwindles to a thin bed 

 of sandstone, sprinkled with a few pebbles, its whole thickness 

 amounting generally to only twenty or thirty feet. There is a 

 corresponding and quite as strildng a diminution in its constit- 

 uent fragments, the pebbles in the most southeastern belts of the 

 formation being often as large as a hen's egg; while in the north- 

 western, their diameter is reduced to that of a pea. 



A similar gi'adation obtains in the thickness and coarseness of 

 nearly all the interstratified sandstones and other mechanical 

 members of the formation. I conceive that this interesting fact, 

 fully established by the surveys of Pennsylvania and Virginia, 

 shows beyond all question, that the southeast was the quarter 

 whence the coarser materials of the coal rocks were derived. But 

 there are not wanting other proofs that the ancient land lay in 

 that direction. These will be presently detailed in describing the 

 gradations witnessed in the limestones and beds of coal. The 

 above general law of distribution, relates, it should be observed, 

 only to the coarser mechanical aggregates, since there are some 

 apparent exceptions to its generality, among the finer-grained 

 slates and shales. Though the texture of these continues to grow 

 finer as we advance westward, some of the strata, when individ- 

 ually traced, seem to increase for a certain distance in thickness. 

 This curious circumstance, which belongs indeed to many of the 

 more argillaceous members of our Appalachian formations, so far 

 from invalidating the above inferences respecting the westward 

 transportation of the sediments, comes beautifully to confirm them, 

 since it is evident, that until a current, holding in suspension a 

 quantity of sedimentary matter, dec-linos in velocity to a certain 

 point, it cannot let fall any considerable amount of the smaller 

 particles. After it has reached a given degree of retardation, the 



