264 _ THE CAUSE OF CRATERIFORM 
by subterranean erosion, and hasten to send you a little paper 
which singularly fits in with some of your views.” With the 
letter was inclosed the reprint of a paper by PRoFEssoR LEBOUR, 
from Zhe Natural History Transactions of Northumberland, Dur- 
ham, and Newcastle-upon-Tyne, vol. xi., part 2, on ‘‘ Certain 
Surface-Features of the Glacial Deposits of the Tyne Valley.” 
He writes: ‘‘I have been led of late to pay special attention 
to the variable, but by no means small, outflow of water, which, 
by means of these many springs, takes place from the land to 
the river (Tyne.) Considering the incoherent character of much 
of the (glacial) deposits, and the steepness of the bluffs, it is not 
easy to note with accuracy how much sediment from within the 
hill is carried away by the springs, or to distinguish such 
sediment from the exactly similar material which is merely 
washed down after the issue of the water. By repeated obser- 
vation, however, both during drought and after heavy rains, 
and at different seasons of the year, I have satisfied myself 
that a very notable amount of sand and very fine gravel, is 
continually being discharged at very varying rates from under- 
ground In other words, it is clear that the sands and gravels 
are being actively removed from within, as well as denuded 
from without.” 
It will be at once evident that PRorEssor LEBOUR’s indepen- 
dent observations fully confirm my theory of the ‘‘ Subterranean 
Erosion” of the Glacial Drift. PRorrssor LEBouR adds, that 
‘Tt seems undeniable that underground erosion, acting in the 
manner indicated, is quite capable of producing hollows in the 
superficial deposits having all the characters of ‘Kettle-holes.’” 
“ True Kettle-holes,” he continues, ‘‘ are held by glacialists 
to be highly characteristic of kames, and of terminal moraines. 
They are regarded as original surface-features left when the ice 
under which they were moulded last melted. 
“That the bowl-shaped depressions of the surface of the 
Sand and Gravel Drift of the Tyne Valley, as exhibited near 
Corbridge, are now in process of constant formation and 
destruction, and that they therefore cannot be in any way 
attributed to ice action—this, I think, is sufficiently proven. The 
point is not unimportant, since on it hinges to some extent the 
question of the amount of denudation which has taken place 
since glacial times, and the date of the last great ice-age. I 
cannot regard any part of the present surface-contour of the 
drift that so largely fills our larger Northumbrian valleys, as 
original—z.e., as practically unchanged in form since the 
disappearance of the glaciers. 
‘“‘ Incidentally, I think a second point has been established, 
viz., that an anticlinal or arched-bedding in drift sands and 
gravels, such as is often stated to be typical of kames and eskers, 
may also be caused by slipping, due to the eroding action of 
water below ground.” 
— a. a 
