304 Dr. CO. Ricketts—Oseillation of the Earth’s Crust. 
rests horizontally on eroded edges of greatly contorted Silurian strata 
which, at the commencement of its formation, were situated at the 
then sea-level, their fragments, consisting of large blocks (some 
being 8 or even 5 feet in their longest diameter, with pebbles and 
smaller particles entering largely into the composition of the lower 
beds of limestone.? 
The curvatures in the Silurian rocks having been formed previous 
to the commencement of the deposition of the limestone, it is evident 
the agency which caused them has had no influence in producing 
the present elevation of the land. The limestone, which is 600 feet 
thick, continued to subside under the accumulation of later Carbon- 
iferous strata, of the Yoredales, Millstone Grit, and Coal-measures, 
which has been estimated to reach a depth of at least 3500 feet. 
North of the Great Craven fault, not only have all these strata been 
removed by denudation, but the Carboniferous Limestone forms a 
plateau, now raised to about 1400 feet above the sea-level; the 
elevated ridges resting conformably upon it; such as Penigent 
(2273 feet), Ingleborough (2375 feet), and Whernside (2414 feet), 
being mere remnants of strata, formed subsequently to the limestone, 
which have resisted destruction. 
Another illustration may be given of subsidence occurring whilst 
deposition was in progress, and of rising of the land during 
denudation. The metamorphic rocks which constitute the Malvern 
range were sculptured nearly to their present contour, but formed an 
island during the deposition of Upper Silurian strata; this was 
demonstrated by Miss A. Phillips, whose discovery proved that 
their base consisted of ‘‘a conglomerate full of Silurian shells, the 
pebbles being fragments of the rock-masses of the Malvern Hills.” * 
Deposition progressed, and the land must have sunk toa great extent 
to permit the formation of the Upper Silurian and Old Red Sandstone 
strata, so greatly developed in the neighbourhood. But these, in 
a much contorted state, were again uplifted, and at the same time 
greatly denuded; but the elevation or formation of the hill is in 
nowise dependent on the foldings of these rocks, for the ridge again 
stood forth as an island from what may have been a near approach 
to the sea-level of the time, when, upon the upturned and eroded 
edges of these Silurian and other rocks, conglomeratic breccia,’ and — 
sandstones were deposited during the New Red Sandstone period, 
depression of the land occurring simultaneously. The land has 
since undergone a great amount of denudation, and the district has 
been raised to its present elevation. Such or similar methods of 
interpretation appear to be generally applicable in assigning a cause 
for the formation of mountains or other elevated ground. 
The cause more generally assigned for subsidence of the crust of 
the earth is secular cooling of its mass. This cannot also account 
1 On a group of Slate Rocks, between the Rivers Lune and Wharfe, by John 
Phillips, F.G.S.; Trans. Geol. Soc. second series, vol. ii. pp. 10 and 13. 
2 Memoirs of the Geological Survey, The Malvern Hills, etc., by John Phillips, 
F.R.S., vol. ii. part 1, p. 66. 
3 Haffield Conglomerate of Phillips. 
