Physical Geography. 137 



thick masses of limestone, but only thin bands, inter- 

 stratified with thick deposits of shale and sandstone, 

 similar in most respects to those of the Coal-measures 

 of Wales, and, like these, interstratified with beds of 

 coal. The inference is obvious, that in these areas the 

 conditions that prevailed were such, that a given area 

 during oscillations of level was at one time sea, as 

 proved by the sea shells in the strata, at another fresh 

 water, as witnessed by the shells Anthracosia, Anthra- 

 comya, &c., and at another time land, as shown by the 

 beds of coal, each underlaid by its terrestrial soil of 

 underclay with Stigmaria, the roots of Sigillaria. 



If this be true, we get a hint of a new phase of the 

 physical geography of an epoch immediately succeeding 

 that of the Old Eed Sandstone. I have often thought 

 that if we might imagine the vast flat territory of 

 Northern Asia, with all its mighty rivers, to face south, 

 so that they might run into a sub-tropical sea, we 

 would have something like a picture of our Carboni- 

 ferous epoch, succeeding one, the chief character of 

 which, was the presence of numbers of large continental 

 lakes. This at all events seems certain, that beds of 

 coal are not the result of woody matter drifted into, 

 and waterlogged in, lake hollows, by rivers, as was once 

 imagined ; but rather, considering the magnitude of 

 the areas which the beds of coal cover, that they bear 

 witness to the existence of a vast continent, or, if we 

 take the whole world into account, of vast continents, 

 through which wandering rivers traversed flat areas, 

 comparable to those of the largest river areas of the 

 living world. Deltas of the present day offer many 

 analogies. The mouth of the Whang-ho or Yellow 

 river is now 250 miles north from where it entered the 

 sea about twenty years ago. The modern delta of the 



