648 STKATIGRAPHICAL GEOLOGY 



present time, for the gulf was completely filled with, salt water, 

 and from the abundance of Littorina littorea and L. rudis in the 

 grey clays left by these waters it has been called the Littorina Sea. 

 The present conditions of brackish water with a limited and 

 stunted fauna have been produced by a second elevation of the area. 

 The Ancylus Lake did not extend so far west as Christiania, 

 where the succession of shelly clays has been described by Sars, 

 Kjerulf, ()yen, and Brogger 2S ; the last-named recognising the 

 following series : 



p , f6. Scrobicularia Clays with S. piperata. 

 Pi. 08 ' l \ 5- Tapes Beds with T. pullastra. 



1 (4. Cardium and Ostrea Beds with C. edule. 



f3. Mya Beds with Mya truncata, Mytilus edulis, Cyprina 

 Late I islandica, Tellina baltica, etc. 



Glacial | 2. Area Clays with Area glacialis and Yoldia lenticula. 

 1.1. Yoldia Clays with Y. arclica, Leda pernula, etc. 



Of these numbers 1 and 2 were formed during subsidence and 

 correspond with the Yoldia Clays of Sweden. The Mya Beds were 

 formed during the subsequent elevation and occur at a little over 

 200 metres above the sea (660 feet). The newer deposits range 

 from 80 metres (260 feet) to sea-level, and the land appears to 

 have been rising all the time. 



F. SOME PHASES OF PLEISTOCENE GEOGRAPHY 



The history of the Glacial epoch has yet to be written, for 

 geologists are not yet agreed as to the interpretation of the records 

 on which such a history must be based, or as to the causes which 

 produced such a great change in the climate of Europe. 



So far as we know there are only three sets of causes which 

 could have wrought such a change of climate ; these are (1) cosmical 

 causes affecting the whole globe ; (2) geographical changes 

 involving the connection or disconnection of continents, and the 

 consequent divergence or introduction of marine currents ; (3) 

 alterations in the arrangement of the areas of high and low 

 barometric pressure, affecting the direction of prevalent winds. 



The cosmical causes have been advocated by the late James 

 Croll, who published a book on the Cause of the Ice-age in 

 1891, but his views have been controverted by Professor Newcombe 

 and Mr. E. P. Culverwell. The possible geographical causes were 

 discussed by Lyell, who argued that the elevation of land in the 

 Arctic regions would tend to refrigerate the northern hemisphere, 

 and that if continuous land existed between Labrador and the 

 British Isles the climate of Northern Europe would probably 

 become colder. Others- again have suggested geographical changes 



