the relative Age of Tertiary Deposits. 5 



period assigned to their existence if the comparison be drawn 

 between them and MoUusca. In this way erroneous inferences 

 with regard to the comparative duration of species, and other de- 

 ductions of an equally important nature, might originate 50/^/^ 

 in an improper identification of the crag beds with one another. 

 I have made use of the above example as a means of showing 

 how desirable it is that there should, if possible, be a right 

 understanding as to the age of the coralline crag: but at the 

 same time 1 would observe that Mr. Lyeli's line of reasoning 

 is one which should be applied with the utmost caution ; for 

 though it may be quite true that the remains of the mammoth 

 have been found, as at Northcliff in Yorkshire, in conjunction 

 with recent species of Testacea^ yet before we can with justice 

 found any argument upon the fact of their occurrence in the 

 same deposit, it is absolutely requisite to show that association 

 is a tolerably conclusive proof of contemporaneous existence : 

 and having settled this point, (which is often no very easy mat- 

 ter, as will be seen in another part of this paper,) we must next 

 inquire whether there be evidence o^ anterior coexistence du- 

 ring periods of equal duration. The fossil elephant of York- 

 shire is found in the red crag, one of the older pliocene de- 

 posits ; but the recent species of MoUusca with which in one 

 case it was associated are not to be traced back to a period of 

 corresponding antiquity. The duration which we are war- 

 ranted in assigninor to these latter is the time which has 



... 

 elapsed since the formation of the Yorkshire deposit, while 



we can date the existence of the elephant from the deposition 

 of the red crag up to the period of its subsequent occurrence 

 in the above-mentioned locality. 



So far then as the progress of geology is concerned, I think 

 ample reasons exist for prosecuting an inquiry into the rela- 

 tive ages of these tertiary beds; but the attainment of that ob- 

 ject through the medium of numerical calculations involves 

 the application of principles, the adaptation of which to the prac- 

 tical purposes of the geologist is an operation complicated in 

 its nature, and which may also be often fallacious in its results. 



Mr. Lyeli's views upon this subject are so well known and 

 have been so generally received, that without entering upon 

 any detail respecting them, I may at once proceed to discuss 

 the considerations which have led me to distrust the value 

 the per-centage test in those instances where we require some 

 thing more than a general approximation towards accurate 

 conclusions*. 



* Some of the following facts and observations were drawn up as a con- 

 tinuation of a paper on this subject which appeared in the Supplement to 

 the Phil. Mag. for June 1836. The delay in the publication has arisen 

 from my wisiiing to lay them before the late^j^ting of the British Associa- 

 tion at Brifatol. 



