Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/78

 hypothesis of a fall in the sea-level, in the author's opinion, accords better with the facts described than Professor Forbes's supposition of changes in the level of the land and sea-bottom.

If Mr. Tyler's hypothesis is correct, that there has been a fall of the sea-level of 600 feet during the Glacial period, followed by an equivalent rise, we ought to find evidence of dry land, of rivers, or, at least, of littoral conditions on the bottom of the sea within the 100-fathom line of soundings.

We should not expect to find a very continuous and unbroken land-surface preserved, as in the upward movement of the sea much ground would be covered with deposits of clay, and shingle, and sand, and much of the old surface removed by currents and waves.

Sir Henry De la Beche, Professor Edward Forbes, and Mr. Godwin-Austen have investigated the present condition of the sea-bottom round the British Isles ; and in their writings are to be found many observations of facts that may be as conveniently explained by the hypothesis of the change in the sea-level as by that of a change in the level of the land and sea-bottom.

Taking the facts as they stand in the writings of these authors, who have all treated the subject most skilfully, the hypothesis of a gradual fall in the sea-level of 600 feet, the author thinks, explains them equally well or better than that of local elevations and then depressions to exactly the same point. The great difficulty in Professor Forbes's argument was to get the Scandinavian flora across the sea. It was necessary to suppose that the space now occupied by the German Ocean was elevated (of which there is no proof), in order to provide dry land for the plants to pass over. The Glacial Mollusca were found by Professor Forbes living in the deeper portions of the German Ocean, which, the author thinks, showed that there could have been no great elevation of the sea-bottom. Whether the sea- level were reduced or the land raised 600 feet, would have the same effect in producing changes of climate and increased excavating -power of rivers ; and he thinks that all the phenomena of a great northern river receiving the waters of the Rhine, Thames, Humber, &c. would occur as described by Mr. Godwin- Austen, if the sea-level were depressed 600 feet.

Mr. Gwyn Jeffreys has lately recorded the discovery of specimens of fossil Arctic shells off the Shetland Isles in about 90 fathoms water. The following species were found by him in dredging, and are arranged in the order of their abundance : —

Terebratula Spitzbergensis. Rhynchonella psittacea. Pecten Islandicus. Tellina calcarea. Mya truncata, yar. Uddevallensis. Trochus cinereus. Molleria costulata. Trophon clathratus. Columbella haliaeeti. Pleurotoma pyramidalis.

All these shells are found fossil in Sweden, and living in the extreme Arctic seas. None of these species are ever found in deep water ; so that their presence, scattered over a wide area of sea- bottom, is remarkable, and corresponds with the discovery of shingle and littoral shells in the English Channel at similar depths. The