628 Original Articles. [Oct., 



to a large extent in genera ; consequently it would be extremely diffi- 

 cult to correlate the littoral accumulations of one region with the deep- 

 sea deposits of another. This is only one of the many sources of 

 complication that render the problem of the relative age of distant 

 deposits so very difficult of solution, and that conspire to place Caution 

 at the head of the cardinal virtues in the code of morality recognized 

 by palaeontologists. 



Philosophical geologists, who cannot but admit the weight of the 

 arguments that have been put forth in favour of the doubtfulness of 

 the relative age of strata apparently contemporaneous, but who are 

 still unwilling to admit such an element of uncertainty into their 

 reasonings, for fear that it would prove a kind of inductive guillotine 

 always ready to fall and cut off their otherwise legitimate inferences, 

 take refuge in an assertion that may be stated in the words of 

 Professor Eamsay, one of the most eminent and philosophical of 

 the geologists who cling to the old notion ; he says, " It appears to 

 me, however, that such reasoning is in error, simply because the 

 reasoner is apt, unintentionally, to consider a whole formation, 

 perhaps from 1,000 to 7,000 feet thick (as in the case of the Bala 

 beds and Hudson River group), as if it were a bed or a thin set of beds 

 representing a particular sea-bottom at a particular time, whereas the 

 Bala beds represent a great many thousands of sea-bottoms more or 

 less regularly piled on each other very slowly. The question must 

 therefore arise, in connection with duration of species, whether under 

 any circumstances the possible time, for instance, that it might have 

 taken to transmit species from the English to the American area is 

 likely to be comparable to the amount of time represented by the 

 interval between the lowest and the highest Bala strata, or even of a 

 longer period."* The gist of which statement is, that the lapse of 

 geological time, which takes place during the deposition of a form- 

 ation like the Bala beds, is something so overwhelmingly great in 

 comparison with that required for the emigration of a fauna to a great 

 distance, that the occurrence of the emigration during the deposition 

 of such a formation would have no appreciable effect on the true, in 

 relation to the apparent, relative ages of even a minor subdivision of 

 them. But I have afready hinted in this article (p. 624) that the 

 recent East Indian Mollusca are so clearly related to European 

 Miocene species that deposits now being accumulated near the shores 

 of the Indian Ocean might easily in future be mistaken for Miocene, 

 and Dr. Duncan has conclusively shown that European Miocene 

 Corals are distinctly East Indian in their affinities. Now the 

 Miocene formation may be roughly considered to be nearly of an 

 equivalent thickness to that of the Bala beds, and many of these East 

 Indian shells occur in its lowest subdivisions. But of more recent 

 date we have the whole of the Pliocene and younger deposits, which 

 attain, probably, in places, an aggregate thickness of 4,000 or 5,000 

 feet more. And yet, notwithstanding this great mass of material that 

 has been deposited since the Miocene fauna first appeared in Europe, 



* Address Geol. Soc, 1863, p. 25. 



