Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/376

Rh species not previously met with, as Tellina fabula and solidula, Donax trunculus, Astarte borealis, and Murex erinaceus. The data are, however, not sufficiently complete to enable us to say more than that the fauna of the epoch during which that deposit was formed presented general features of a decidedly mixed Celtic and northern character.

Putting the mammaliferous crag aside, it is evident that the "coralline" and "red" crags afford clear indications of a state of things in the seas in which they were formed, very distinct from that now presented by the seas surrounding the British Isles. Mr. Searles Wood's reference of the coralline crag fauna to a type comparable with that now presented by the assemblage of marine animals in the Mediterranean, or on the coast of Portugal, appears to me to come very near the true state of the case; and his observatitms on the subject, in the ninth volume of the Annals of Natural History, are well worthy the attention of geologists. The epoch of the red crag was evidently marked by a new set of conditions which materially changed the character of the fauna. The key to the problem presented by these must be sought for in the zoological phenomena exhibited by the remarkable strata known under the names of "Boulder clay," "Arctic or northern drift," "Pleistocene," or sometimes "Newer Pleiocene," and in some authors "post-tertiary," including (in part) the "Till" deposits, which for convenience I shall henceforth mention as glacial, or as beds of the glacial epoch. My chief purpose in drawing up this essay, is to assist in the elucidation of that most interesting and important formation—one which will engage much of the attention of the geological surveyors—by exhibiting the nature and value of the evidence afforded by organic remains found in those beds, and the bearing of that evidence on the history of animated nature within the area under examination by the Survey, immediately before and subsequent to their formation. This evidence has not hitherto been put in a tangible form, nor has it been fully appreciated except by very few geologists, of whom Mr. Smith, of Jordan Hill, was the first to endeavour inductively to work out in detail the many problems it involves, and to impress upon the geological world the variety and interest of the palæontological phenomena presented by the glacial beds in the northern districts of Britain and Ireland. There is now a great mass of evidence existing, scattered through books or known to a few and as yet unpublished—sufficient to enable us to present well-grounded generalizations.

Exactly a century ago, the attention of the greatest of modern naturalists was arrested by the zoological phenomena presented by the glacial beds in Sweden. In the course of his journey through Wast-Gota, in 1746, Linnæus visited Uddevalla, the very locality destined long after, to furnish Mr. Lyell with part of the materials for his celebrated essay