476 Correspondence. 
1 foot long, 5 inches square, and 2 inches thick, and appears to have 
almost entirely filled up the pipe, retaining its square form through- 
out: the whole was deposited in the short space of from six to nine 
months. 
The chemical composition, which is rather remarkable, is as 
follows :— 
(Sp. gr. at 15:5 C., 3646.) 
2 
Sulphate of Baryta : 5 ; ; Oa a 
Ferric Oxide : : : i : : Ol 5:5 
Carbonate of Lime ; ; s : ; 9:46 82 
Alumina A , ; F : : . traces 1-0 
Water A é é . f ; : 2:02 2:0 
99:16 100:3 
No. 1 was from the centre of the deposit, No. 2 from the out- 
side. This analysis was made by a friend. 
The only reference which I have been able to find with regard to 
the occurrence of sulphate of baryta in coal-pits is in a paper by 
Messrs. R. C. Clapham and T. Daglish, read before the British 
Association at Newcastle in 1863. It is there stated that Mr. Foster 
had found quite a large mass: Dr. Richardson also had observed 
it in the waters of Walker Colliery.— Yours truly, 
J. WALLACE YOUNG. 
GLASGOW. 
DENUDATION, UNCONFORMABILITY, AND THE VALE OF CLWYD. 
To the Editor of the GEOLOGICAL MaAGaAziIne. 
Srr,—Geological notions, like all others, occasionally swing pen- 
dulum-like from one extreme to the other. The old controversy 
between the respective importance of Fire and Water has not ceased ; 
though, judging from the number of important results which are now 
attributed to its denuding power, Water is evidently in the ascendant. 
Believing myself that these results are rather over-estimated, I am 
induced to offer to your readers the following remarks. 
A gap in the order of strata—as, for example, Carboniferous Lime- 
stone resting upon Lower Silurian rocks, or the Trias, as in the Vale 
of Clwyd, supposed to rest upon the Carboniferous Limestone—does 
not necessarily imply the denudation of the strata missing from 
between them; for it is possible to conceive that towards the close 
of the deposition (or indeed at any period during its deposition) of 
any group of strata, the deposit might, in any portion, or the whole 
of it, be raised above the sea-level, and so remain, while newer 
deposits were being formed around it; and so a Azatus in the order 
of the strata would be caused, which would be greater or less in 
proportion to the time the relative positions of sea and land remained 
unchanged. 
Thus, suppose in any particular district the sea-bottom to have 
been elevated towards the close of the Lower Silurian period, and 
to have remained above the sea-level until the Old Red Sandstone 
was deposited, and then be again submerged, we should in that 
