66 Dr. J. Croll—On the Cause.of the Glacial Epoch. 
the north of England appear in this district to be replaced by valley. 
deposits of various kinds. 
The present features of East Norfolk have been, I think, in the 
main produced by the deepening of irregularities left on the dis- 
appearance of the great ice-sheet. The mound of Contorted beds 
pushed up by the ice still remains and forms the high land near 
Cromer. Valleys have been cut out of it, and the tops of the 
contortions planed off, but it still forms an important feature in the. 
physical geography of Norfolk. 
I have been quite unable to find any trace of the inter-glacial 
valley erosion mentioned by Messrs. Wood and Harmer. Where 
the Middle Glacial descends into the valleys, it is the result of 
contortion, and not of erosion, and it is difficult to understand how 
a Post-Glacial valley could coincide with an Inter-Glacial one, cut 
through soft beds, and which had been filled up and must be again 
excavated. If a valley happens to cut through a number of 
contortions, synclinals will occasionally bring the gravel to the 
bottom ; but where an anticlinal is cut, the gravel is not to be found, 
and it is then said that it has been all scoured out of the valley. 
IiJ.—Mr. Hit on tue Cause or THE GuaAcrAL Epoca. 
By Jamzs Crott, LL.D., F.R.S. 
HAVE just read Mr. Hill’s criticism on my views as to the cause 
of the Glacial Epoch, and have to thank him for the fair and 
courteous way in which he has treated the subject. I fear Mr. Hill 
has come to his conclusions somewhat too hastily. He appears to 
me to have totally misapprehended the real effect of fogs and a 
snow-covered surface on climate, and aiso the influence of eccentri- 
city on the trade-winds. But I must defer for the present entering 
into any discussion on the matter. My object just now is simply to 
direct attention to an erroneous assumption which appears to lie at 
the base of nearly all the objections which have been urged against 
the theory that the Glacial Hpoch resulted from an increase of ec- 
centricity. 
It is assumed that when the eccentricity was at a high value, and 
the summer in perihelion, more snow must have been melted than at 
present. It is assumed that the quantity of snow melted must be 
proportional to the heat received from the sun. Suppose that on a 
certain area a given amount of snow falls annually. The amount of 
heat received from the sun per annum is computed, and after the 
usual deduction for that cut off by the atmosphere has been made, 
if it be found that the quantity remaining is far more than sufficient 
to melt the snow, it is then assumed that the snow must be melted, 
and that an accumulation of snow and ice year by year in this area 
1s impossible. To one approaching this perplexing subject for the 
first time such an assumption looks very plausible, but nevertheless 
it 1s one totally opposed to known facts. Take, for example, Green- 
land. We know that that area receives from the sun per annum 
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