154 
SCENERY OF SWITZERLAND. 
deposits, though fossil remains are rare in the Swiss 
deposits of this age. 
It would be out of place in the present volume 
to enter into the consideration of the causes which 
probably led to the existence of the glacial period, 
or to its probable date. I have in my " Prehistoric 
Times discussed this question, to which I may refer 
those who wish to go further into the subject, and I 
will here only say that I see no sufficient reason to 
change the opinion (though doubts have recently 
been thrown on it by Sir H. Howorth and others), 
that it was mainly due to astronomical causes, and 
reached its maximum from 50 to 100,000 years ago. 
If this explanation be correct it follows that 
periods of cold and warmth must have followed one 
another more than once, at intervals of 21,000 years. 
And in accordance with this we find, as Morlot long 
ago pointed out, that the glaciers have advanced and 
retreated more than once. 
Beds indicating warmer conditions are interposed 
between glacial deposits, and the Swiss and South 
German geologists believe that there were three 
periods of cold with milder intervals. In Scotland 
James Geikie and others have brought forward evi- 
dence of more numerous oscillations.* 
* The Great Ice Age. 
