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developed, though of course not so deep as they were later on 
in the year. Others were of much less regular form: some had 
the hummock on the south side well developed; others had 
no perceptible hummock, but seemed to have more or less of a 
raised margin all round; others seemed to show a slight hum- 
mock on the north as well as the south side. 
The explanation of the phenomenon which was suggested 
was as follows :—A collection of gravel gives rise to an irregular 
or roughly circular shallow water-hole, the water beimg at first 
at a considerably lower level than the edge of the hole, formed 
in part of snow and soft ice. 
(1) Towards noon the sun’s rays are incident upon the 
surface at a small angle, a comparatively small portion are 
reflected, and the radiations which enter the water are for the 
most part absorbed before reaching the wall of the hole, and 
the resulting heat is carried to the bottom by the descending 
current of dense water, whose temperature has been raised 
above the freezing point. This melts ice at the bottom and 
deepens the hole. On the other hand, towards morning and 
evening the angle of incidence is larger, a larger proportion of 
the rays are reflected and strike upon any portions of the east 
and west boundaries of the hole which are above water. The 
rays which enter the water have a shorter path to traverse in 
water before reaching the wall, and therefore will reach and 
melt it in a greater proportion. The east and west walls will 
thus be more melted than the south wall, and the hole gradu- 
ally assumes its oval shape with its longer axis east and west. 
(2) The ice in the neighbourhood of the holes is for the 
most part of very rough and irregular surface, very pure, and 
with small bright crystalline faces inclined in all directions. 
Consequently a considerable portion of the rays incident on any 
portion of it, and there reflected, will strike another portion of 
the surface, so that the wasting of any portion of the surface is 
due not only to the rays primarily incident on that portion, 
