Trochus cinereus. 

 Molleria costulata. 

 Trophon clathratus. 

 Fleurotoma pyramidaUs. 



398 ■ A. Tyhr — Formation of Deltas. 



fluviatile, and estuary gravel beds (containing fossils), raised beaches 

 and tin-stone gravels covered with marine remains, on account of 

 their different organic contents. As far as regards position merely, 

 all these deposits are now on one horizon, and the circumstance of 

 some beds being deposited before the Glacial Period and before the 

 fall of the sea-level, and others at the close of the Glacial Period, 

 may explain their being on one horizon, and apparently almost con- 

 tinuous, although really separated by an immense interval of time ; 

 no change having taken place in the level of the land, but great 

 oscillation in the sea-level having occurred in the Glacial Period. 



Mr. Gwyn Jeffreys has lately recorded ^ the discovery of specimens 

 of fossil littoral arctic shells off the Shetland Isles, in about 90 

 fathoms of water. The following species were found by him in 

 dredging, and are arranged in the order of their abundance : — 



Terebratella Spitzhergensis. 



Rhynchonella psittacea. 



Fecten Istandicus, 



Tellina calcarea. 



Mya truncata, Tar. Uddevallensis. 

 All these shells, I believe, 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 

 littoral shells in the English Channel are not of Arctic forms, like 

 those in the German Ocean, and this is a proof that they were de- 

 posited 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 9 species of fossil shells, by Mr. Gwyn 

 Jeffreys, at a depth of 90 fathoms, off the Shetland Islands, is an im- 

 portant addition to Forbes' and Godwin-Austen's observations. This 

 discovery affords independent and corroborative proof of identical 

 conditions with those observed by them in other parts of the sea- 

 bottom, and it establishes the existence of littoral conditions near the 

 present 1 00 fathom line during the Quaternary Period in the North Sea, 

 and is therefore an additional support to the hypothesis we are con- 

 sidering. 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 this would involve the summer heat being 130° 

 Fahrenheit near the locality of a mountain range of 1,500 to 2,000 

 feet. The amount of rainfall depends greatly upon the elevation of 

 the 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.* 



^ Brit. Assoc. Eeports, Dundee, 1867, p. 431. 



2 Abstract " On Loess of the Valleys of the South of England and the Somme." 

 Geol. Journ., p. 604, vol. xix., 1863. 



