80 
plateau near Llangollen. I do not remember having seen 
béulder-clay on the surface of the flat limestone rocks of the 
Norber plateau, though the spaces between many of the 
blocks in situ are often partly filled with a kind of grass- 
covered earth which may have resulted from the accumulation 
of the insoluble part of the limestone. On the supposition 
that the boulders were transported by a great sheet of land- 
ice (as Mr. Tiddeman believes), little or no boulder-clay would 
be allowed to gather, according to Professor Geikie (Pre- 
historic Hurope, p. 289). Had the sea, with boulder-laden 
floating ice, swept over the high rocky plateau, there would 
have been still less likelihood of boulder-clay being deposited. 
9. Ttainfall of the Above Plateau.—It may be desirable to 
compare the rainfall of the two plateaux described in this 
paper, especially as in, or near to, the two localities it was 
very nearly the same in 1881. At Austwick, near Norber 
plateau, tt was 41°70 inches, while at Llangollen, it was 42°81 
inches. According to Professor Phillips, the mean annual 
depth of rain, from 1837 to 1850, was 43°35 at Settle (which is 
not very far from the Norber plateau). 
10. Pedestals of Boulders in Ireland.—My attention has 
lately been directed to a work entitled Misswres, Hractures, 
and Faults, by Mr. Kinahan (of the Irish Geological Survey), 
in which he states that on the Arran Islands, where the rain- 
fall is great, the limestone has weathered away from four to 
six inches since the glacial period, as proved by the un- 
weathered pedestals of limestone under the erratic blocks ; 
while inland similar pedestals are seldom three inches in 
height. 
11. Bearing of the Above Facts on the Time which has 
elapsed since the Close of the Glacial Period.—After making a 
series of calculations based on the results of observations 
made in the two districts described in this paper, I was led to 
the conclusion (as already hinted) that the average depth of 
the hollows which have been excavated around boulders by 
the pluvio-torrential or mechanical action of rain-water 
{assisted by its chemical action under favourable conditions) 
is not more than about six inches. With regard to the rate of 
denudation, the extension of many flat rock-surfaces under 
boulders (especially on Norber plateau) shows that it must be 
exceedingly slow. But if we allow a thousand years for the 
excavation of only an inch in depth of the hollows around the 
boulders, this would give us not more than 6,000 years as the 
time which has elapsed since the boulders were left in their 
present positions through the melting of the ice by which 
they were transported. These calculations are only vaguely 
