16L> 
the river, above Netherside, and there is one, tolerably con- 
spicuous on the roadside, rather more than a mile south of 
Kilnsey. 
If these boulders came from Ribblesdale, round by Settle, 
how did they get over the barrier that closes in Upper 
Wharfedale ? What made the ice move northward and 
up the dale, when all the evidence goes to show it was ever}-- 
where else moving southward and down the dale ? 
At that time T saw no solution of the difficulty, but since 
the date of my previous papers, it struck me, now some years 
ago, that I saw a possible explanation. I now conceive, that 
were we able to remove the covering of drift and alluvium, 
that conceals the solid rocks of the valley bottom near 
Kilnsey, we should find Silurians in place in the valley 
bottom. With reference to this idea, it is noteworthy that 
some strong springs break out at the foot of Kilnsey Crag. 
This, though it does not prove, itself suggests and favours the 
idea that we are noa?' the base of the limestone. There is 
also a certain portion of the lower part of Littondale, in which 
no solid rock is seen in the valley bottom, being entirely hidden 
by drift, screes, or alluvium, along which two strong springs 
break out at, or near, the base of the limestone scars. 
Briefly, I would suggest that somewhere in the lower part of 
Upper Wharfedale, or less likely, in the lower part of Litton- 
dale, Silurian rocks exist in the valley bottom beneath the 
covering of superficial detritus, and that it was from this 
outcrop of rock that the Silurian boulders, found so excep- 
tionally in the limited part of Wharfedale, indicated above, 
were derived. It will be observed that all the Silurian 
boulders occur south of, and below the area, which, from the 
existence of strong springs may be supposed to be at the 
base of the Carboniferous rocks. 
This supposition, that Silurian rocks exist below the 
alluvium and drift beneath Kilnsey Crag, or, for the matter 
