366 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 7, 



Had the Devonshire slates contained Silurian fossils on the one 

 hand, or Lias fossils on the other, they would, as a matter of 

 course, have been referred to the one or the other of those two for- 

 mations. 



The choice now, however, is limited to the period of the Old Eed 

 Sandstone and that of the Carboniferous Limestone, unless we as- 

 sign to them a hitherto undefined period between those two, or sup- 

 pose them to be older than the Old Eed Sandstone. 



We need hardly trouble ourselves, however, with the two latter 

 suppositions. 



Now, the Devonian fossils are said to comprise some species 

 which are found also in Silurian rocks *, and are not known in the 

 Carboniferous Limestone. They undoubtedly contain many spe- 

 cies found in the Carboniferous Limestone f which are not known 

 in any Silurian rock, as well as a few (such as Strophomena rhom- 

 hoidalis), which range from Silurian into the Carboniferous Lime- 

 stone. 



The Devonian fossils also comprise some peculiar species of Silurian 

 genera, which do not occur as species in Silurian, nor as genera in the 

 Carboniferous Limestone. Some of the Trilobites are conspicuous 

 examples of this case. 



They also comprise some peculiar species of Carboniferous ge- 

 nera {ProduGta, for instance) which do not occur as species in Car- 

 boniferous Limestone, nor as genera in the Silurian rocks. 



Lastly, there are some genera (such as Stringocephalus among the 

 Brachiopoda, Megalodon among the Conchifera^ and Oalceola, what- 

 ever that may be) which are peculiarly Devonian genera. 

 - Altogether, the Devonian fossils have an intermediate aspect 

 (whether in the specific characters of some, as pointed out first by 

 Mr. Lonsdale, or in their general fades) between those of the Upper 

 Silurian rocks and those of the Carboniferous Limestone. 



The natural presumption, therefore, was, that the beds contain- 

 ing those fossils were intermediate in age, and therefore contempo- 

 raneous with the Old Eed Sandstone, which could be observed stra- 

 tigraphically to hold that intermediate position. 



However natural that presumption was, however, I must be per- 

 mitted to urge that it was a presumption only, and not a proof. 



The intermediate biological character of the Devonian fossils, to 

 whatever extent it may exist, is neither a conclusive proof that they 

 are of the age of the Old Eed Sandstone, nor that they were not 

 contemporaneous with those in the Carboniferous Limestone. 



The geological age of fossils is proved by the stratigraphical po- 

 sition of the beds containing them. When that age has been 

 established by reference to clear sections in one or two locaHties, 

 the fossils themselves are admitted as evidence of the age of beds 

 in other localities whose stratigraphical position perhaps is not 

 clear. In all such cases, however, there must be either an avowed 



* Mr. Etheridge reckons these as 13, mostly Corals, 

 t Mr. Etheridge reckons these as 92. 



