THE WEST OF ENGLAND AND BAST OF WALES. 431 
a permanent obstruction to further transportation. Initial con- 
centrations are not so easily explained; but the conjecture may be 
hazarded that much of the ice which was able to launch the boulders 
would be incapable of carrying them beyond a small distance. It 
may likewise be supposed that the parent mountain would protect 
many of the previously launched boulders in front of it from being 
carried away, by its interfering with the straight-onward course of 
currents of air or water or (as some would believe) land-ice. In- 
stances of such concentrations, and also of intermediate concentrations 
arising from local obstructions, will be described in the sequel. 
Most of the erratic small stones in the northern part of the boulder- 
strewn area consist of ‘Silurian grit” from the Lake-district and 
south of Scotland; and in the plain of Cheshire and Lancashire they 
are much more frequently flattened and uniformly grooved than any 
stones of which I can hear in any part of the world, not. excepting 
the neighbourhood of the winter-quarters of the late Arctic expe- 
dition*. Among the larger boulders those derived from felspathic 
rocks generally predominate, excepting in the granite concentrations. 
Criffel and Eskdale granite come next in order of frequency ; and we 
accordingly find that felstone and granite rocks 7m sitw are the most 
lable to break up into large blocks. In the above boulder-area 
(away from the Lake-district) I have not seen a single instance of 
a perched block. 
Il. Tee creat KigxcuDBRIGHTSHIRE GRANITE-DISPERSION. 
1. Boulder-supplying Capacity of Criffel—Initial Concentration. | 
—It is probable that many small Silurian-grit erratics were dis- 
persed from the south of Scotland ; but how many it is impossible to 
say, because (as Prof. Ramsay tells us) they are often quite undis- 
tinguishable from Lake-district Silurian grits. The exposures of 
compact felstone are so limited on the south coast of Scotland as to 
render it improbable that many of the large felstone blocks of the 
northern drift-area came from that quarter. It must, however, 
have sent off a considerable number of large blocks of metamorphic 
rocks and a few of Silurian grit and slate, as they may be found 
scattered as far south as the terminal concentration west and north 
of Wolverhampton. The granite-dispersing capacity of Kirkcud- 
brightshire must have been very great. ‘There are three granite 
masses many miles in diameter, but Criffel mountain and the neigh- 
bourhood would appear to have sent off the great majority of the 
boulders. Many parts of the mountain are now covered with loose 
granite blocks, and the configuration of the ground may haye ad- 
mitted of small glaciers, on the sides of which angular blocks may 
have fallen. Nearly all the Criffel blocks (as already stated) are 
angular or subangular, excepting where an approximately spheroidal 
structure probably existed ; and they may have been transported on 
* Mr. De Rance, F.G.S., who examined the stones brought home by Capt. 
Feilden, informs me that they were not so distinctly glaciated as those of the 
Cheshire and Lancashire plain. 
Q.J.G.8. No. 139. Qn 
