316 PROCEEDINGS OF THE GEOLOGICAL society. [Apr. 19, 
striking case, and has most kindly communicated to me the details, 
which will immediately appear in his work*: near South Barrule 
there is a hillock formed of granite, quite different in nature from any 
other rock in the island; this mass of granite is about three-fourths 
of a mile square, and is 757 feet above the level of the sea; from 
this pot the boulders are thickly dispersed to the south-west, and 
they can be continuously followed up to a height of 788 feet above 
the summit of the present boss. Mr. R. Mallet informs me that 
facts of a similar nature have been observed in Ireland. More 
striking cases occur in the United States, in New England, in 
New York, and in northern Pennsylvania. Prof. Hitchcock 
observes}, that the Silurian rocks of New York and the quartz 
in the vaileys of western Massachusetts have undoubtedly been 
carried over and left upon the Hoosac and Taconic mountains, at a 
height of ‘‘ upwards of one thousand or two thousand feet.” Lastly, 
I may mention the analogous case of the chalk-flints, associated with 
boulders of various kinds, observed by the Dean of Westminster and 
myself on Moel Tryfan, at the height of 1392 feet above the level of 
the sea, and which (as well as the chalk-flints at the intermediate 
point of the Isle of Mant) there is good reason to believe must have 
come from Ireland, and therefore, at least in the case of North Wales, 
from a considerably lower level. 
The first pomt to consider is whether, in these several instances, 
the boulders have really come from a lower level, or whether they 
may not (and I am indebted to Sir H. De la Beche for this caution) 
have been derived from strata now entirely denuded, but which for- 
merly extended up to the same level with the boulders. Or secondly, 
whether the boulders, after havimg been deposited, may not have been ~ 
raised by an unequal elevatory movement above their parent district, 
or the district itself have been depressed by subsidence below them. 
With respect to the former supposed greater extension and subsequent 
denudation of the parent rock,—in such cases as those near Kdin- 
burgh it is possible that this may be sufficient to account for the 
phenomenon. Where the boulders are of granite, as at Glen Roy and 
‘the Isle of Man, this view implies that a mass of that rock has been 
worn down, equalling in thickness the difference in level between the 
existing mass in situ and the boulders: in North America, where the 
boulders lie from 1500 to 2000 feet above their source, the denuda- 
tion on this view must have been immense, and it must all have been 
effected within the glacial period, as the low country is covered with 
boulders ; this likewise is the case with the boss of granite in the 
isle of Man. Can it be supposed with any probability that the 
chalk-formation formerly extended in Ireland up to a height of nearly 
1400 feet? In the case of the boulders described by Prof. Phillips, 
I am assured by him that the above view is quite madmissible ; and 
he has pointed out to me conclusive reasons, but which, considering 
* The Isle of Man, its History, &c., by the Rev. J. G. Cumming. 
+ Geology of Massachusetts, vol. i. (Postscript, p. 5a), and Address to Associa- 
tion of American Geologists, 1841. 
¢ The Rey. J. G. Cumming in Transactions of British Association, 1848, p. 61. 
Oa oe 
