Correspondence. AU7 
district miss the whole of the Upper Silurian rocks; and the Old 
Red Sandstone and the Carboniferous Limestone would rest directly 
upon the Lower Silurian strata, just as it is seen to do on the west 
of Oswestry. Of course, the land so raised would become subject to 
the influence of the atmosphere and rain, and its outlines would 
become modified in proportion to the power of these agents and the 
extent of surface so exposed. ; 
The thinness, too, of a ‘Formation’ at a given point does not 
necessarily imply a previous ‘ erosion’ or denudation of the strata at 
that point. ‘For it is but reasonable, and in accordance with what 
we know, to suppose that while the deposition of matter may be the 
same in character over an extended area, it may yet differ greatly 
at various points in the raée at which it is deposited, liable as the 
deposition is to be affected by currents, by the nature of the solids 
supplying the materials, by the presence and intensity, or otherwise, 
of sub-marine springs, &e. Thus, whilst over the large area sup- 
posed we should find beds similar in their general character, and 
occupying the same stratigraphical position, we should also find local 
variation as to colour, texture, and thickness. 
It apears to me also, that Unconformability of strata does not 
necessarily imply a lengthened lapse of time, or a change in the 
mineral or organic constituents of the deposit. It is easy and 
reasonable, as it appears to me, to conceive that, during the forma- 
tion of any deposit, the sea-bottom in any portion of it may be tilted 
up from a nearly horizontal to a highly inclined position, and the 
work of deposition go on again with scarcely any interruption; and 
thus we should have strata of the same age (geologically speaking ) 
resting unconformably on each other, while, on the other hand, a 
much older ‘formation’ which had been elevated, as I have before 
supposed, and remained so, whilst newer deposits were forming 
about it, may again be depressed, and in such a position as that the 
new matter shall be deposited conformably to it. 
It appears to me that no theory of Denudation or Unconforma- 
bility is universally applicable, but both are liable to be affected by 
local and particular causes: hence the need of great and constant 
discrimination in all geological theorizing. To some of your readers 
these remarks may appear mere truisms; nevertheless I do think that 
they are too often forgotten. ‘Thus, in the August Number of this 
Magazine, Mr. Maw argues from the supposed aggregate thinness of 
the Carboniferous Limestone near the head of the Vale of Clwyd, 
in favour of a great erosion of that formation prior to the deposition 
of the Trias; whereas, if the foregoing remarks are true, the sup- 
posed erosion is not necessarily consequent upon the thinness of the 
Limestone. And then we find no less an authority than Mr. Jukes, 
in his lecture at Birmingham before the British Association, as- 
suming it as an axiom that where the Carboniferous Limestone is 
present, it was once covered by the Coal-measures. Here, again, 
your readers will see that while this might have been the case, and 
probably in some instances was so, it does not follow logically that 
any group of strata now exposed on the surface was once overlain 
by the group next in geological order. Though, singularly enough, 
