THE GEOLOGY OF PUFFIN ISLAND. 89 
the Humber to the heights overlooking the valley of the 
Thames; to this view there are, however, serious objec- 
tions, and the chalky boulder clay was probably formed by 
the melting of the southernmost tongue of ice at a period 
somewhat earlier than the explanation would admit. The 
theory of extra moraine lakes, due originally to Belt, has 
been applied with greater probability to the drifts of the 
valley of the Thames.* 
T'o this theory probably the greatest objection is that it 
postulates the power of ice to ascend 800 feet, to which 
the deposits are lowered by the admitted submergence of 
500 feet, and this many English geologists declare to be 
impossible, or at least unproved. In this case it is of 
course useless to cite, as some objectors do, the Swiss 
glaciers in their present development, as they do not 
represent conditions analogous to those of an ice sheet 
such as we have here to consider. It must be admitted 
that there is no well authenticated case of an ice sheet 
to-day ascending slopes for any considerable distance ; 
in Greenland such probably occurs, as in the opinion 
of the Danish explorers the country is saucer-shaped, 
consisting of a deep central depression surrounded by 
a border of mountains over which the ice has to ascend 
during its route to the sea. That ice has done so in the 
past, is more easily proved, e.g. by the case of the old 
Rhone glacier, or to cite an English case, that described 
by Mr. Goodchild in his paper on the Eden valley, where 
blocks of “‘brockram,” a breccia peculiar to the valley 
below the 700 foot contour, have been carried across the 
Stainmoor Pass, over a height of at least 1350 feet, while 
other rocks have been carried up for over 1000 feet. 
Belt has called attention (‘‘ Nature,” vol. x., p. 63) to an 
admirable instance of this action within the L.M.B.C. 
* Goodchild, Proc. Geol. Assoc., vol. ix., pp. 155 and 156. 
