Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/90

 Globigerina in the cold area, while the extent of the other differences loses much force by the identity of the Mollusca. We know not also whether there is not a passage from one area to the other. We require therefore more evidence before the geological value of the distinction of the two areas can be fully accepted ; at the same time the importance and interest of such an influencing cause must be kept in view.

I will now say a few words on one of the most important bearings that these deep-sea researches have on chronological geology. Objections have been taken on various grounds to the percentage test of Sir Charles Lyell, as evidence of relative age. The data of the deep-sea dredgings furnish us with curious and apparently paradoxical results and such as might seem fatal to this test. Suppose an isolated portion of the deep-sea Atlantic bed had been elevated at some late period, and that we were yet ignorant, as we were only twelve years since, of what was to be found in the unexplored depths of the ocean. Suppose further that the Atlantic deposit had taken place on such rocks as the Palaeozoic strata of Cornwall or South Ireland. A chalky-looking deposit would then have been found overlying old rocks, with nothing to indicate stratigraphically its geological position, and with fossils to a great extent new. In the absence of a complete knowledge of the deep Atlantic fauna, I will take, as a specimen of what they might have been, the result of one deep dredging in 5964 feet. Mr. Jeffreys obtained in this single dredging 186 species of Mollusca. Of these he found : —

91 species recent or living.

24 „ formerly known as fossil only, and belonging to the Pliocene strata of Sicily ; some of these are undescribed.

71 ,, new or undescribed.

186

The conclusion would have been that 95 out of the 186, or 51 per cent., were of extinct species ; and of these, 24 would be referred to Pliocene age. What would have been the inference as to the age of the beds ? Certainly, on palaeontological evidence alone, there could have been but one conclusion. They must have been classed as Pliocene or older, although these researches have now shown all the species to be recent.

The case, however, is an extreme and exceptional one. It is true that, in future speculations, the possibility of such a case happening must be taken into consideration ; but the depths of the Atlantic are so great that, unless in case of a disturbance such as that of the elevation of the Alps or the Andes, we are not likely