308 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



First mid second Points of Difference. — From the examination 

 which the author and other geologists have made of the several fos- 

 siliferous formations of North Wales, described in his paper, and 

 those of the English border counties, he draws the conclusion, that 

 the same formation contains fewer fossils in the one district than in 

 the other, according as its thickness is greater in the one district than 

 in the other ; and this conclusion he supports by several examples. 



This seeming correspondence between the paucity of fossils in a 

 formation in any given district, and its thickness in that district, the 

 author conceives to be deducible, as a consequence, from the laws 

 which govern the distribution of marine animals in the sea at differ- 

 ent depths, which have been derived by Professor E. Forbes from 

 his dredging operations in the ^Egean Sea. 



The laws to which he more particularly alludes are, 1st, that the 

 number of species, and of the individuals of every such species in- 

 habiting the sea at any given depth, is the less as that depth is 

 greater, and the greater as that depth is less ; and, 2ndly, that the 

 range of depth inhabited by any species is the greater as the depth is 

 greater, and the less as the depth is less. Whence it follows, that 

 were an internal sea to be gradually filled up, and were the solid 

 contents of the basin so filled up to be divided into strata, each stra- 

 tum being determined by the identity of the species therein pre- 

 served, the thicker strata, containing the fewest organic remains, 

 would be found towards the bottom, and each successive stratum in 

 ascending order would be thinner and thinner, and more and more 

 replete with fossils. 



Among the examples which he gives of an existing correspond- 

 ence between the thickness of a formation and its comparative defi- 

 ciency in fossils, is the Wenlock series, as seen in Worcestershire 

 and in North Wales. This series, near Llangollen, is about 3500 feet 

 thick, and is not known to have afforded more than a dozen differ- 

 ent fossil species. Of these the most abundant occur in the middle 

 and lower beds, and belong to the genus Creseis. The author 

 states, on the authority of Professor E. Forbes, that the remains of 

 the recent species of this genus are found in great abundance in the 

 Mediterranean, in the sediment at the bottom of the sea at great 

 depths ; but that in shallow bottoms such remains are rare. The 

 same series near Dudley does not attain, perhaps, a fourth of the 

 above thickness ; and it is crowded with organic remains, among 

 which the stony corals abound ; and these are specially indicative of 

 a deposit in shallow water. The same may be remarked of the 

 Lower Silurian formations, which in North Wales are far thicker and 

 contain fewer fossils than in Shropshire and the border counties. 



The author has not pointed out any criteria by which to distin- 

 guish the Cambrian series from the Lower Silurians, unless it be 

 that the former series lies below the latter, and is almost, if not 

 wholly, destitute of organic remains. The law of diminution in the 

 living animals of the sea as the depth increases, points to a total 

 absence of animal life at about 300 fathoms below the surface. The 

 Cambrian series may therefore be regarded, either as a deposit 



