Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/89

 of a nature unsuited for the existence of life over the original seabed. It is now evident that the absence of life in the depths of the AEgean is due to the fine tenacious mud (which, by the by, E. Forbes likened to chalk), in the same way that those areas of the Mediterranean, discovered by Capt. Spratt, and of the Atlantic at the entrance of the Straits of Gibraltar, discovered in the ' Porcupine ' expedition, to be covered by fine mud, apparently in a state of continual slow deposition, were found to be almost entirely barren. On the other hand, where the rocks consist of sandy strata, any fossils composed of carbonate of lime may have been dissolved out, and all traces of them lost by the percolation of rain-water, after their elevation into dry land, as happens in the Bagshot Sands, in which it is only by chance in the few instances where the sand happens to be consolidated by a ferruginous cement that the impressions and casts of shells are preserved. Another well-known cause for the absence of fossils in a sedimentary deposit is the circumstance of the strata having undergone metamorphic action. I should hardly have thought it necessary to mention these various causes to account for non-fossiliferous rocks, but for a recently expressed opinion of a presumed more general acceptance of Forbes's hypothesis amongst geologists than has been at all the case.

As bearing also upon the distribution of life in the same stratum at points in near proximity, Dr. Carpenter notices that there are areas in the North Atlantic in which the temperature varies considerably at the same relative depths ; and he infers that there are permanent warm and cold areas, distinguishable not only by differences of from 10° to 15° of temperature, but also by a difference of marine life, such as might present a geological difficulty. He notes the presence of Globigerinoe and abundance of vitreous sponges on a fine muddy bottom in the one, and of northern forms of Echinodermata and Crustacea on a bed of sand and stones in the other. Mr. Jeffreys, however, did not find the same difference in the Mollusca. He states that the result of his examination shows that there are forty-four species in the warm area and fifty-five species in the cold area, and these latter included all the forty-four of the former ; and he accounts for the absence of Globigerina on the ground that " the strength of the submarine current in the cold area is sufficient to sweep away and remove these slight and delicate organisms," which, from later observations by other naturalists and himself, he believes inhabit only the superficial stratum of the sea. The slight difference in temperature seems hardly sufficient to account for the absence of