In the year 1851 I read a paper before the Geological Society “On the Superficial Accumulations and Surface-markings in North Wales,” in which I attempted to show that there had been two glacier epochs in that country, one before, and the other after the deposition of the boulder drift, which was ploughed out of some of the larger valleys by the secondhand smaller set of glaciers; and in a later work on the old glaciers of North Wales, I went further, showing that cold sufficient to form glaciers lasted during the whole time of submergence and emergence, both when the higher mountain-tops stood out of the sea as a cluster ofsmall islands, and afterwards when the whole land rose out of the water.

The first of these memoirs touched on several subjects not immediately connected with the glaciation of Wales, though bearing in a larger sense on the same Geological period, and on the same set of questions. This the Council of the Geological Society decided not to print in their Journal, on the ground that it was too speculative—an opinion with which, in a great measure, I now coincide. One question was, however, raised in this unprinted matter which I do not yet remember to have seen in any published paper, and I now mention it because of late the attention of many persons have been more and more drawn to the discussion of the subject.