TOE WESL OF ENGLAND AND EAST OF WALES. 429 
rising) the cold and ice decreased northwards. This accords with 
the fact that many of the boulders which must have gone over 
ereat heights are found far south, and likewise with the fact that 
the boulders in the latest of the deposits left by the great submer- 
gence gradually die out until they disappear in the low-level sands of 
the Cheshire and Lancashire plain. It is probable that the maximum 
of cold (during the submergence) was not reached until the land 
had sunk to a considerable depth. It must have taken much thick 
ice to transport the myriads of large granite blocks found as far 
south as Wolverhampton—blocks which could not have reached their 
destination without a submergence of at least from 500 to 650 feet. 
It must have taken still thicker ice to have transported the larger 
Arenig boulders now found around the Clent and Lickey hills, 
boulders which could not have cleared the Welsh mountains under a 
less submergence than 1000 feet, at a time when the “cold wall” 
had been pushed to its extreme southerly position. This low tem- 
perature may possibly have continued until the submergence had 
reached about 1400 feet, the reason for this suggestion being that 
the shells found on the summit of Moel-y-Tryfaen (near Caernarvon) 
indicate a colder climate than any shells found at a lower level in 
the western part of South Britain (see Mr. Shone’s paper, Quart. 
Journ. Geol. Soc. for May 1878, p. 393)*. I do not, however, wish 
to attach too much value to this suggestion. 
8. Causes of Radiating or Fan-shaped Boulder-dispersions.—All 
the great boulder-dispersions, and some of the smaller ones, are more 
or less fan-shaped ; in other words, they have radiated from an area 
much narrower than their terminal breadth. This radiation has 
occurred not only where the parent area is backed by lower ground, 
but likewise where it is backed by higher ground, and by ground 
approximately on the same level. The form of the ground, there- 
fore, has had little to do with the radiating character of the disper- 
sions. The ground behind the parent area is generally destitute, or 
nearly destitute, of boulders derived from that area. This fact has 
been adyanced as a proof that the distributing agent must have been 
land-ice, the persistent forward movement of which prevented any 
dispersion in a contrary or backward direction. The same effect, 
however, may have been produced by a persistent ice-laden current 
of water, or by winds mainly blowing from the same point of the 
compass, but changing their directions within certain limits during 
the submergence or emergence of the land. It is perhaps worthy of 
remark, though too much importance ought not to be attached to 
the fact, that the direction of the main part of the Arenig dispersion 
coincides with that of the anti-trade winds. The Shap-fell granite t 
* ‘The gravel-and-sand (with boulders) very near the highest point of Moel- 
y-Tryfaen has yielded the following Arctic and Scandinavian shells, which have 
not yet been found at a lower level in the drifts of Cheshire, Lancashire, &e. :— 
Astarte depressa (erebricostata) ; Natica affinis ; Trophon clathratus, var. scalari- 
Jormis, and var. Gunneri. 
+ Having already, along with other geologists, written on the Shap-fell dis- 
persion, I shall at present only direct attention to the discovery of the ¢erminal 
concentration (with a few stragglers further south) of a stream of Shap-fell 
