442 SKETCH TOWARDS A 



might have occurred at the depression of the former, 

 and antecedent to the deposition of the chalk, must 

 remain a question, till the interval between the upper- 

 most lignite and the chalk is better known than it 

 seems yet to be. In the preceding one, at least, supra- 

 marine rocks are degraded, through a long period, but 

 marine animals do not exist, or have become rare. If 

 I formerly called this a great blank in the Theory or' 

 the Earth, I cannot but consider the present view as 

 proved : while, under a revolution of this character, 

 we ought perhaps to expect what has not yet been 

 found ; discordance of position, as in other cases, and 

 this now forming the real blank. If the general rela- 

 tion of the chalk to the green sand, marks, like that of 

 the oolithe to the red marl, the gradual increase of 

 life, after extinction, I need but slightly repeat, that 

 the clays, sands, and gravels often found above it, must 

 have been superior marine deposits, whether they are 

 now rocky strata or not, and that it is not therefore 

 the last. I have left little to say respecting the lignite 

 coals. Except in some few cases of transportation, 

 and in others,, where they have been derived from 

 marine plants, their origin is similar to that of the in- 

 ferior one, if perhaps they are always the produce of 

 shores or aestuaries, while the other is often lacustral. 

 Taking as our type, the great floats of wood brought 

 down by the American rivers, we can conjecture re- 

 specting those of transportation : but this source is 

 excluded whenever they are repeated with intermediate 

 rocky strata, and with vegetable fragments and beds of 

 shells, marking a regular stratification under repose ; 

 as it is also, under the great extent which many occupy. 

 But for the certainty of this, the former view of revo- 

 lutions and conditions, from the coal to the chalk, 

 would be a fable: while in this case, as in the inferior 



