806 Prof. T. McKenny Hughes—Brecciated Beds at St. Davids. 
It appears that Mr. T. F. Jamieson, in 1865, was the first who 
attributed this depression in different countries during the Glacial 
period “to the enormous weight of ice thrown upon the land; and 
considered that the melting of the ice would account for the rising 
of the land which seems to have followed the decrease of the 
glaciers.” * 
Professor Shaler, of Harvard College, U. 8. A., in 1874, also 
appears to have arrived independently at the same conclusions 
respecting the cause of subsidence during the Glacial period; he 
considers that “we may more reasonably look to the weight of ice 
accumulated on the continents for the depression of the land areas it 
occupied than to any other cause.” ” 
Similar explanations, in which Mr. Fisher coincides, may account 
for the subsidence recently occurring in Greenland simultaneously 
with a rapid increase of accumulated snow ; and also for the gradual — 
rise of Scandinavia as a natural result of the recession of glaciers. 
In all the older rocks the same fact is evident :—wherever any 
large amount of strata has been laid down, there has likewise been a 
persistent depression, which progressed in at least an equal degree 
with the accumulation. In each individual formation, throughout the 
whole series, from the Laurentian rocks of Canada, or the Longmynd 
rocks of Shropshire, up to those now in process of deposition, the 
record in the one is the record in them all,—it was formed during a 
period of subsidence. 
Though there has thus occurred a simultaneous and progressive 
depression during the deposition of the accumulations constituting 
stratified rocks, it might be argued that the subsidence has taken 
place to the same extent over the whole area, and that the strata are 
thicker in one place than in another simply because their bases have 
been deposited on surfaces of unequal elevation, that is, on areas con- 
sisting of submerged valleys and hills. It requires a certain stratum, 
forming as it were a base-area, traceable over extensive districts, and 
situated, when formed at or about the same level, before the relative 
amount of subsequent deposition and subsidence can be calculated 
in different portions of the district in question. The Carboniferous 
formation affords most satisfactory examples for determining the 
problem. 
(To be continued in our next Number.) 
V.—On tHE Breccratep Bep 1n THE Dimetian at Sr. Davis. 
By Tuos. M‘Kenny Hvuenss, M.A., 
Woodwardian Professor of Geology, Cambridge. 
i hn recent discussions upon the classification of the Archean, great 
importance has naturally been attached to the occurrence of beds 
of fragmental origin. In the distinctly voleanic beds these are 
numerous; in the older or more highly metamorphosed portion of 
1 History of the last Geological changes in Scotland, by Thomas F. Jamieson, 
F.G.S., Quart. Journ. Geol. Soc. vol. xxi. p. 178. 
2 Recent Changes on the Coast of Maine, by N. S. Shaler, Memoirs Boston Soc. 
Nat. Hist. vol. ii. 
3 Cause of the Glacial Period, by Charles Ricketts, Grox. Maa. Dee. II. Vol. 
II. 1875, p. 573; Rey. O. Fisher, op. cit. pp. 223 and 224. 
