Giants' Kettles near Oiristiania a)idin Lneerne. — UpJiam. 297 
coal mining, their bottoms being in the coal bed. When 
the drift filling the one first discovered was cleared out, it 
was found to be 38 feet deep, with a diameter of about 15 feet 
at the bottom, increasing to a maximum of 42 feet and a min- 
imum of 24 feet across its top. The second pot-hols, of sim- 
ilar basal diameter in the coal bed, had not been chared of 
its drift contents; but it is known from the levelling and test- 
pits or borings of the mining company, to have a depth of 
about 50 feet in the rock, with a covering of 15 feet of drift 
above.* 
According to these observations and records of glacial 
pot-holes, in their best known localities of both Europe and 
America, it seems to me most probable that the time of 
their excavation in nearly all cases was the early part of the 
Glacial period, or some stage of glacirl extension, when the 
ice-sheet was being formed upon the land by snowfall. On 
any hilly country the ice must have attained an average depth 
somewhat exceeding the altitude of the hills above the ad- 
joining lowlands before any general motion of the ice-sheet 
could begin. During the process of slow accumulation of 
the ice-sheet, the summer melting upon its neve surface would 
produce multitudes of rills, rivulets, and brooks, which might 
unite into a large stream, and this, pouring through a cre- 
vasse and melting out a cylindric nioulin, might fall perhaps 
100 or 200 feet or more on a moderately hilly region, but 
probably sometimes 500 feet or more on a mountainous dis- 
trict, while yet the ice motion, though sufficient to permit the 
formation of the crevasse, might not have gained a definite 
current to carry the crevasse, moulin. and waterfall away from 
the spot where they were first formed. We may thus explain 
the continuation of a glacial waterfall in one place while it was 
excavating one of these giants' kettles. After the ice-sheet 
acquired a current because of the greater thickness and pres- 
sure of its mass, such deep cylindric excavations in the bed- 
rock could not be made; and during the recession and final 
dissolution of the ice-sheet it seems probable that its receding 
border had steeper gradients and consequently even more 
*Geol. Survey of Pa., Annual Report for 1885, pp. 615-625, with a 
map, sections, and two plates (views from photographs of the first pot- 
liole when cleared out for use as an air shaft.) 
