THEORY OF THE EARTH. 425 



terrestrial period, as there did in the case of the infe- 

 rior coal. The lias, with its carboniferous surface, 

 must have been depressed beneath the sea, or the su- 

 perior marine strata could not have been formed ; and 

 thus there follows a tenth condition of the earth, under, 

 of course, the usual doubts as to its extent, during which 

 this vegetable stratum also is becoming coal. If such 

 a depression were sudden, it would be termed a revo- 

 lution : I know not whether the term is worth a dis- 

 pute, were it a gradual operation. 



At any rate, the marine strata then formed, extend 

 to the carboniferous surface in the green sand, being 

 the place of the second great lignite. And there is 

 here a renewal of the same difficulties as in the pre- 

 ceding case. Was that surface brought up to the light 

 by a revolution of elevation, or it did simply reach it 

 by the increase of the strata? Geology must seek for 

 proofs, as in the former case ; I possess no evidence, 

 and am ready to believe whatever is proved : yet, in 

 the mean time, following the general analogy, this 

 should be an eleventh condition of the Earth. And, 

 there is yet another difficulty, renewed for the third 

 time. There are marine strata forming while the plants 

 for this last lignite are growing; and we cannot dis- 

 tinguish them. In any of the three cases, how are 

 these strata, parallel in time, to be known ? They are 

 riot superimposed on the several coals, nor can they 

 be subjacent. Inferior, or nearer to the centre of the 

 earth, they must originally have been ; but if we cannot 

 tell where to seek them now, this at least is plain, that 

 the theories of superposition and succession among the 

 later strata, which even I have been obliged to follow, 

 are mere hypotheses, and that our ignorance of these 

 is infinitely greater than geologists had ever suspected. 

 Thence probably are there many other cases of infe- 



