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of Wharfedale. These boulders are foreigners, as no Silurian 
rocks exist in place in any part of Wharfedale, at the surface 
at least, and, being foreigners, they form an exception to the 
general character of the Drift of the Dales. Oenerally speak- 
ing, the Drift of the Dales contains no blocks or pebbles of 
any other rocks, than such as are found in situ, within the 
limits of the various Dales themselves. 
Thus, in Wensleydale, at least below Hawes, no boulders 
are found except of carboniferous rocks ; the same is the case 
in Coverdale, Bishopdale, Semmerdale, and Nidderdale. In 
Ribblesdale we have, besides Carboniferous boulders, Silurian 
ones as well, derived from the outcrop of Silurian rocks, 
which is well known to occur in that dale near Horton. In 
Dentdale, too, I have not seen any but carboniferous boulders 
in all that part of the dale which lies above the Silurian strata. 
The same is the case in Littondale, and also, with the single 
exception I am going to describe, in Wharfedale. 
Wharfedale consists of two distinct portions, in neither of 
which do any but carboniferous rocks appear at the surface. 
From its source in the recesses of the Fells, to the southern 
edge of Grass Wood, near Grassington, the river Wharfe runs 
in a narrow valley. This forms the upper part of Wharfedale. 
From Grass Wood to Burnsall, the river is bounded by Fells 
on the east side, but on the west by an open, drift-covered 
country, which stretches away westward beyond the river 
Ribble ; in fact, between Grass Wood and Burnsall, the dale 
is discontinuous. Below Burnsall, the river again flows in a 
narrow dale to below Otley. In this lower part of the dale 
none but Carboniferous rocks exist, and no other rocks are 
found in the Drift or as boulders. Between Burnsall and 
Chapel House, however, Silurian Erratics are very plentiful. 
It is true, they have been mostly cleared off the surface of 
the land and built into the walls, where they are plentifully 
used as throughs but in the walls they are numerous, so 
