251 
Laughton, &c, eastward of which no detritus could readily 
have been drifted ; hence only magnesian limestone detritus 
is found to be carried eastward over certain parts of Notts. 
This magnesian limestone gravel, as I said before, reposes 
upon the old sea gravel peculiar to Notts, and therefore the 
latter is older. 
Now leaving the origin, &c, of the northern drift, as 
explained by Trimmer and Cumming, I have to account for 
the three or four east and west lines of gravel, as well as the 
absence of the northern drift from the Yorkshire and Derby- 
shire coal fields, and from certain parts of Notts. 
Taking for granted that the greatest part of Great Britain 
has been submerged, some parts to the extent of 1,600 feet, 
we cannot exempt the Midland Counties of Yorkshire, 
Derbyshire, and Nottinghamshire. But why is there no 
incursion of Cumberland boulders into this district, since 
southwards, up to the Severn, and into North Wales, on one 
side of the Penine chain, and again through Norfolk up to 
the Thames, on the eastern side, the whole surface is covered 
with this diluvium ? Now it has been proved by Trimmer 
that in Wales, whenever a high bluff-land or range of hills 
presents itself to the north, to the south of such range the 
northern drift has not reached. The Snowdonian chain of 
hills has effectually protected Cardigan Bay (Crieccieth). 
Harlech Castle, as well as the high lands in Anglesea, have 
protected the Menai Straits. Also the Penine chain of hills, 
which separates Lancashire and Yorkshire, although ranging 
north and south, has never been crossed by this diluvium; and, 
therefore, I conclude, in like manner, the elevated millstone 
grit ranges in the north of Yorkshire, as around Wharfdale, 
Swaledale, and Airedale, between Brough and Harrogate, 
have protected and acted as a dam to any irruption from the 
north ; and hence the absence of such detritus in the Yorkshire 
and Derbyshire coal fields, and of the great part of Notts. 
