492 



tions of the Silurian system. When this point was considered, I 

 expressed to the Society ray opinion, in common with Mr. Mur- 

 chison, as to the insufficiency of the proofs relied on by our 

 Foreign Secretary, and we felt that we had a right to call for more 

 conclusive evidence. The simple fact of shales having been found 

 charged with true coal plants, raised so strong a presumption in 

 favour of their belonging to the regular carboniferous series, that 

 the burthen of proof rested with him who wished to assign to them 

 either a higher or lower position. Our scepticism was regarded 

 by Mr. Greenough as implying too marked a bias for a preconceived 

 theory, and this he afterwards hinted in his Anniversary Address*. 

 I may affirm, however, that in the first place it implied on my part 

 no distrust of Mr, De la Beche's skill or experience in geological sur- 

 veying, and that had Professor Sedgwick and Mr. Murchison ad- 

 vanced a similar opinion on analogous proofs, I should equally have 

 withheld my assent. Suppose, for example, they had announced to 

 us that they had found fossil fruits and leaves identical with those of 

 Sheppey in strata of the age of the white chalk with flints. I should 

 have demanded from them, in corroboration, the most clear, un- 

 equivocal, and overwhelming evidence. If it were a region of dis- 

 turbed and vertical strata, I should expect them first to have re- 

 sorted in vain to every hypothesis of inverted stratification with a 

 view of explaining away such an exception to the general rule. 



I might perhaps be told that we are unacquainted with the flora 

 of the upper cretaceous period, and I admit that we are as ignorant 

 of it as of that which belonged to the transition period, but when 

 we consider the contrast of the shells and other fossils of the 

 chalk and London clay, we naturally anticipate that if plants are ever 

 found of the precise age of our chalk with flints, they will not prove 

 to be of the same species as those of the Sheppey clay. There 

 is a like presumption from analogy against the conclusion that the 

 same vegetation continued to flourish on the earth from the period 

 of the lower grey v>'acke to that of the coal, because we know that 

 in the course of the intervening epochs the testacea, zoophytes, fish, 

 and other classes of organic beings were several times changed. 



In regard, to the proofs relied on by Mr. De la Beche, I should 

 observe that he never attempted to show that the plant-bearing 

 * Proceedings Geol. Soc, vol. ii. p. 164. 



