X.— On the relative Ages of the Tertiary and Post-tertiary Deposits of the Basin of the Clyde.

Abstract

In a former paper* I described the indications which I had observed of changes in the level of the sea in the basin of the Clyde, which must have taken place at times, geologically speaking, extremely recent, as the indications are met with above the Till or erratic block deposit; and in a subsequent paper † I stated, that from the Arctic character of several shells which have not been found recent in the British Isles, that there was strong probable evidence that the climate was colder at the period of their deposition than it is at present. Having given a detailed account of my observations, with catalogues of the recent and fossil shells of that district to the Wernerian Society (Wern. Mem. vol. viii.), I confine myself, at present, to the results of observations made subsequently, which afford the most satisfactory evidence that in these comparatively modern deposits, there are two distinct formations differing in climate and fauna, and separated by a wide interval of time. If a difference in the testaceous fauna characterises the tertiary period, the oldest of these, which contains about fifteen per cent, of extinct or unknown species, must be held as a tertiary formation, and belongs to the newer Pliocene or Pleistocene of Lyell, and the newest, a post-tertiary one, as all the shells hitherto discovered, agree with those of our present seas. Both of these deposits are anterior to the recent or historical period. We have therefore, in the superficial or earthy covering