1866. | Geology and Palxontology. 415 
Croll’s notion of an ice-cap is one of the mgeniously devised loop- 
holes of escape, which command admiration and deserve success. 
They obtain the first im abundance, the second in a very slight 
degree. 
”'The March number of the ‘ Philosophical Magazine’ contained a 
very elaborate mathematical paper by Mr. Heath, in which he showed 
that if Mr. Croll’s theory “ cannot explain elevations or depressions 
of 1,000 feet, it does teach us there is an agency at work in nature, 
which had perhaps been overlooked, which must be. borne in mind 
in all speculations where tens of feet are material.” We are very 
glad that Mr. Croll can be said to have done so much service inci- 
dentally. In the April number, however, Mr. Croll makes an 
onslaught on Mr. Heath, restates his theory of an ice-cap, and 
recalls one or two recantations he had previously made. His paper 
is followed by a note by Professor W. Thomson, who gives a for- 
mula for the calculation of the depression caused by the flow of 
water to the pole in consequence of the ice-cap altering the centre of 
gravity. 
In the May number of the same periodical Mr. Croll was 
vigorously attacked by Mr. J. Carrick Moore and _ Professor 
Haughton; two assailants of undoubted power, both mathemati- 
cians and both geologists—a rare combination. It is quite obvious 
that the ice which Mr. Croll supposes to have “ capped” the North 
Pole durmg the Glacial Period must have been derived either 
entirely from that now existing round the South Pole, or more or 
less from the water of the present seas. Mr. Carrick Moore 
assumes, in the first place, that the ice was obtained at the expense 
of the sea, and shows that an ice-cap, as supposed by Mr. Croll, 
of 7,000 feet thickness at the pole, would, on this view, cause at 
lat. 60°, a depression of the sea level, not an elevation, to the extent 
of 833 feet, in consequence of the drain of sea-water exceeding the 
elevation caused by the attraction of the ice-cap to that extent. 
Secondly, he proves that if the ice which formerly existed in the 
northern hemisphere had been derived from the southern, and has 
now returned there, an ice-cap of a uniform thickness of 2,000 feet 
over the Antarctic regions, which is certainly an exaggerated esti- 
mate of the existing ice, would not supply one-twentieth part of 
that required to cause a submersion of the land at lat. 60° to the 
extent of 2,300 feet. Finally, Mr. Carrick Moore observes that, 
“as the quantity of ice to be supplied by the melting of that at the 
south pole is so greatly disproportionate to its object, it is 
unnecessary to discuss what appears to me to have been too lightly 
assumed—viz. that when one pole is under glacial conditions, the 
opposite will be entirely free from ice.” 
With respect to Mr. Croll’s notion of the eccentricity of the 
earth’s orbit being connected with changes of climate, the Rey. 
