i) 
‘channels. But it is necessary to guard the observer against 
mistaking pre-glacial excavations for the post-glacial channels 
of streams, because the upper boulder-clay of England and 
Wales has been generally deposited as a wrapper which, in a 
thin bed, rises to the summits of the knolls and goes down to 
the bottoms of the valleys which must have been excavated in 
glacial or pre-glacial times. According to Mr. De Rance, in 
aereat part of Lancashire, glacial deposits occupy old pre- 
glacial valleys, producing the phenomenon of valley within 
valley. Mr. Searles V. Wood tells me that in Holderness, 
north of Hull, there are many examples of streams making 
channels in a mere wrapper of glacial drift which follows the 
undulations of the surface of the ground. 
4. Time Indicated by the Vertical Hutent of the Pedestals of 
Boulders.—Boulders may be said to be without pedestals when 
the rock-surfaces on which they rest extend continuously 
under them with little or no change of level; in which case 
the boulders, owing to their particular forms or positions, are 
not capable, to an appreciable extent, of concentrating or 
intensifying the action of rain water, which, in a diffused 
state, would appear to exert scarcely any denuding influence 
around the boulders. They may be credited with acquired 
pedestals where the underlying supports have been caused by 
the pluvio-torrential action resulting from wind-blown rain, 
and by the form and extent of the water-collecting surface 
furnished by the boulders. They may be said to rest on 
appropriated or usurped pedestals, where the latter depend 
cen the previous removal by denudation of the surrounding 
rock, in which case they are merely ‘‘ perched blocks,” and 
can therefore furnish no evidence of the vertical extent of 
circumjacent denudation which has taken place since the 
boulders came into their present positions. 
5. Boulders on Limestone Rocks North of Llangollen—Distri- 
bution and Deseription.—About two and a half miles north of 
Llangollen, a ravine above Brook House leads up to a high 
limestone plateau on the left called Craig-yr-ogof. On then 
going north towards a ravine called Nant-hen-Gastell, many 
boulders of Arenig felsite may be seen on grass-, fern-, or 
-heath-covered surfaces, with a hollow on one or two sides 
caused by the down-splashing of concentrated rain-water from 
the boulders. After crossing this ravine, and turning to the 
left as far as the brink of the innermost well-defined cliff- line, 
a monstrous-looking boulder of nearly black felsite, about 
eight by seven by ‘five feet in diameter, may be seen in a 
perched position close to the brink. It has a rather irregular 
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