tn 



REPORT — 187G. 



the same survival of catastrophisni — the onion-coat theory, as Herbert Spencer 

 named it. 



The Geological Survey has corrected its tables in one important direction : it 

 lias shown the contemporaneity of imlike groups in different parts of Britain, the 

 distinct types of the Old Red Sandstone, Carboniferous, Permian, and Purbecks 

 being placed in parallel columns. To some extent this is a curtailment of the 

 thickness of the rock series, the dissimilar strata are not piled on each other. But 

 the curtailment might be carried still further. The marine and terrestrial con- 

 ditions are simultaneous : if we could identify the dry land for each deep sea, we 

 should have possibly the overlap of periods producing extraordinary combinations, 

 though not perhaps Mesozoic and PaliBozoic faunas contemporaneous. But the 

 British series may be tabulated as follows : — 



Zand Surfaces. 



Coal Measm'es. 

 Permian. 



Trias. 



Purbeck. 

 Wealden. 



Miocene. 



Pleistocene. 



Laeiistrine ^" Fluviatile. 



Cambrian. 



Old Red Sandstone. 



Calciferous Sandstones. 



Marine. 



Laureutian ? 

 Silurian. 



Carboniferous Limestone. 



Juraasic. 



Neocomian. 



Cretaceous, 



In the case of the Cretaceous series, Mr. Ramsaj' has given illustration of the 

 ingenious views of De La Beche regarding the contemporaneity of deposits super- 

 posed one on the other. The Lower Greensand is contemporaneous with part of 

 the Chalk ; so -were parts of the Wealden : nay, even of the Purbecks a portion 

 must have been forming while the Cretaceous sea was gradually deepening 

 southward and eastward. 



It may be said that the recognition of the parallelism would not make very much 

 difference after all — that it would not one whit lessen the time spent in forming 500 

 feet of rock to know that there was elsewhere another .500 feet formed at the same 

 time. But the shortening of the geological list by striking out the overlaps of the 

 formations, and thus counting them only once, is of itself a matter of some conse- 

 quence, since, the maximum thickness of the Cretaceous being nearly 3000 feet and 

 that of the Weald 1.500 feet, even the partial coincidence in time of these masses 

 would, on Mr. CroU's calculation of 1 foot of deposit per thousand years, make 

 a considerable difference in tlie chronology, still more if the Carboniferous Lime- 

 stone be set against its probable contemporaries the Upper Old Red Sandstone and 

 Coal-Measures. Mr. Jukes's bold erasure of the Devonians was of itself a veiy im- 

 portant change in the chronological table ; and I doubt not others may yet be 

 achieved. But, it may be said, the Cretaceous still rests on the Wealden ; the ver- 

 tical thickness still remains. But is the ordinary method of estimating the thick- 

 ness quite reliable ? In some cases, as in the productive Coal-Measures, there is 

 tolerable uniformity ; but among the lower Coals and the Mesozoic strata, where the 

 strata or groups of strata are not regular, the maximum thicknesses of all are, as has 

 been already shown, apt to be taken ; and thus an aggregate more or less in excess 

 of the real thickness results. 



But, recumng to an objection already referred to, arrange it as you like, you get, 

 say in Wales, a known thickness of 50,000 feet. But the rocks there are tilted; and 

 the absolute depth which they attain in this position isunkno^vn. In North Ame- 

 rica the Laurentians are estimated at 30,000 feet ; but though there is eveiy reason to 

 believe that they have not been covered to any extent with later deposits, the total 

 thickness of the sedimentary crust is, for the same reason as in Wales, imknown. 

 Bigsby has shown how varied are the surfaces on which the later deposits are laid 

 down ; how gxeat therefore must be the deductions from the sum total of maxi- 

 mum or even, average thickness of all formations before we approximate to the 

 actual thickness of sedimentary deposits at any one point. But take the actual 



