238 On computing the relative Age of Deposits 



lence is necessary to remove fossils from one formation, and 

 to deposit them in another. Hence, all fine-grained deposits 

 are beyond the limits of the above error, and they form the 

 largest portion of the whole stratified series. No one can 

 doubt that the fossils of the carboniferous limestone, the lias, 

 the chalk, and the London clay are contemporary with the 

 deposition of these strata respectively ; for a current suffi- 

 ciently strong to bring these fossils from a distance would 

 not have allowed the fine-grained matter which now envelopes 

 them to be deposited. 



Moreover, the fossils of an older formation can hardly 

 become enveloped in a newer one, unless the two deposits are 

 unconformable. The older deposit must be raised up, so as 

 to form sea cliffs and projecting rocks, before its destruction 

 can take place; for we can scarcely suppose that the sea ever 

 denudes its own bed to such a depth as to lay bare and extract 

 the fossils of a preceding and extinct creation. Hence, when 

 we have a long series of conformable deposits, as in the 

 Silurian or oolitic rocks of England, there can be no doubt 

 that the various groups of fossils are contemporary with the 

 beds which they characterise, even though the sandy or 

 gravelly structure of some of these beds may indicate the 

 flow of submarine currents. I am not aware whether the 

 red crag can be shown to be unconformable to the coralline 

 crag ; but, if it cannot, I should be inclined to think that all 

 the tertiary fossils found in the red crag are contemporaneous 

 with its deposition. 



Again, in the cases where an admixture of fossils of dif- 

 ferent ages has unquestionably taken place, there can rarely 

 be any difficulty in distinguishing the genuine ones from the 

 erratic* To take the examples above cited in the Farring- 

 don sponge-bed, the Belenmites, which I suppose to be derived 

 from the coral rag, are all worn and broken, while the sponges 

 andTerebratulae indigenous to the spot are in the most delicate 

 state of preservation. Again, the secondary fossils found by 



* " It may be said that these older shells, entering into new deposits, carry 

 with them evidence of the stratum from which they have been derived, or 

 that, at all events, their worn appearance would distinguish them from the 

 more recent Molliisca with which they are associated. This is so far from 

 being the case, that considerably finer and more perfect specimens of the 

 Voluta Lamberti can be picked up from the sea shore, where they have 

 been dashed by the waves on a shingly beach, than can ever be obtained 

 from the beds of the crag formation itself. In fact, this gradual process 

 of degradation appears to be of all others the most favourable for detaching 

 organic remains from the matrix in which they are embedded." {Charles- 

 worth in Proceedings of the British Association for the Advancement of 

 Science, Loudoit's Mag. of Nat. Hist., vol. ix. p. 540.) — Edit, 



