Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/79

 littoral shells of the English Channel are not Arctic species, as are those in the German Ocean ; and this fact is a proof that the former were deposited on the sea-bottom of the English Channel before the junction of the English Channel and German Ocean at the Straits of Dover was effected.

The discovery of these nine species of fossil shells by Mr. Gwyn Jeffreys at a depth of 90 fathoms, off the Shetland Islands, is an important addition to Forbes's and Godwin-Austen's observations. This discovery affords independent and corroborative proof of identical conditions with those observed by these authors in other parts of the sea-bottom ; and it establishes the existence of littoral conditions in the Quaternary period near the present 100 -fathom line in the North Sea, and is therefore an additional support to the hypothesis we are considering.

A fall in the sea-level of 600 feet would not only produce littoral conditions off Shetland, without any change of level of the sea- bottom, but would tend to lower the temperature of the air very much, and also to increase the rainfall. There are certain conditions under which a rainfall of 300 inches per annum might be produced in our climate ; but they would involve the summer heat being 130° Fahrenheit near the locality of a mountain -range of from 1500 to 2000 feet in height. The amount of rainfall depends greatly upon the high temperature of the air at the sea-level (supposing it saturated with moisture) and the low temperature of the air on the mountain- range intercepting the aerial currents.

We might have in our latitude a summer heat of 130° from the general elevation of the heat of the globe, from an increased volume of the Gulf-stream, and from a greater prevalence of the west and south-west winds.

The pluvial period which the author had previously proposed, and which was so much objected to in the discussion of May last, does not require any greater volume of water than has been before suggested by geologists, as heights of 80 feet were estimated for the ordinary difference of winter and summer floods in passages of two different memoirs by Mr. Prestwich, as occurring during what he considers the earlier part of the gravel-period *.

There is, however, in England no appearance of tropical vegetation in the Quaternary deposits, such as we should expect would accompany a temperature of 130° ; and we must therefore try some other alternative.

We could not have rivers varying 80 feet in summer and winter without some such rainfall, unless we had pluvial and tidal conditions very different from those now in the Thames and Somme Valleys. What we want is to explain the enormous rise of rivers in a cold climate during the Quaternary period.

In the year 1840 the ice brought down by a January flood, gorged at a point about nine miles from the mouth of the Vistula, cut a

the South of England and the Somme, read June 19, 1862, Proceedings of the Royal Society, vol. xii. p. 170.]
 * [See Abstract of a paper, by Mr. Prestwich, on the Loess of the Valleys of