1868.] TYLOR FORMATION 0¥ DELTAS. 9 



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 

 Jeflfreys at a depth of 90 fathoms, off the Shetland Islands, is an 

 important addition to Eorbes's and Godwin-Austen's observations. 

 This discovery affords independent and corroborative proof of iden- 

 tical conditions with those observed by these authors in other parts 

 of the sea-bottom ; and it establishes the existence of littoral con- 

 ditions 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 Avhat 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 accom- 

 pany a temperature of 130° ; and we must therefore try some other 

 alternative. 



We could not have rivers varying 80 feet in summer and winter 

 mthout some such rainfall, unless we had pluvial and tidal con- 

 ditions 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 



* [See Abstract of a paper, by Mr. Prestwich, on the Loess of the Valleys of 

 the South of England and the Somme, read June 19, 1862, Proceedings of the 

 Royal Society, vol. xii. p. 170.] 



