Prof. Hull's lead, divide the Drift into three parts (viz. Lower Boulder-clay, Middle Drift or Interglacial Gravels, and Upper Boulder-clay), I was much struck by the section. If it had occurred in Lancashire there is no doubt it would at once have been set down as a very good example of the "Interglacial Gravels" and "Upper Boulder-clay."
I had not many minutes to examine the section, but I picked up during the time I was there a few of the shells and shell fragments. These I submitted to Mr. R. D. Darbishire, F.G.S., of Manchester, who has determined the following species:—Astarte elliptica, A. compressa, Leda pernula, Mactra elliptica, Natica, and Mytilus.
On comparing these with my list of shells from the Boulder-clay about Liverpool[1], I find that during several years' close search Astarte compressa only occurred in great rarity in the numerous localities I examined; and according to Mr. Darbishire's list it is "very rare" at Blackpool. Astarte elliptica was frequent in some localities and rare in others. Leda pernula, supposed to be a typical arctic shell, was very rare in the localities where it was found.
There is thus, according to shell-evidence, nothing to give an "Interglacial" character to these gravels as compared with either the so-called Upper or Lower Boulder-clays of the north-west of England. My opinion has long been against this tripartite classification, and I have pointed out more than once that it rests upon no intelligible basis[2]. The examination of the Irish Drift still further confirms me in the opinion that the marine Boulder-clays of the north-west of England and Ireland are but phases of one long sequence of events uninterrupted by changes of climate.
Prof. Hull has applied the same classification of "Lower Boulder-clay," "Middle Sands and Gravels," and "Upper Boulder-clay" to the Drift of Ireland[3]. It is quite evident, however, if the Boulder-clays of Galway Bay, as represented by the sections at Blake Hill, and the Fermoy valley in county Clare on the opposite coast, as well as the innumerable islands of Drift in Clew Bay, are members of the "Lower Boulder-clay," it is quite a different thing from the Lower Boulder-clay of Lancashire. The latter is marine, the former contains no evidences whatever of marine conditions.
On the north shores of Belfast Lough, beneath the celebrated raised beach, are to be seen sections of a Boulder-clay that corresponds in appearance with the marine Boulder-clays of the north-west of England. Large glaciated blocks of travelled stone, some of which I could have matched with erratics taken out of the Bootle Dock excavations, are to be seen on the beach washed out of the low cliffs of purple Boulder-clay, which is evidently, as in Cheshire and Lancashire, largely reconstructed from the Triassic Marls. I did not notice any shell-fragments in the Belfast Boulder-clay; but my time was limited, and in Lancashire it often requires close examination to detect them.
