524 Proceedings of the Royal Physical Society. 
ferous sandstones underlying the New Red Rocks, for 
example, ferruginous stains from this source have been traced 
to a depth of 600 feet below the base of the stratum whence 
the colouring matter originally came. In this case, however, 
the abundant diffusion of matter of vegetable origin in the 
Carboniferous Rocks so affected has led to considerable 
modifications of the colouring effect, as might be expected to 
be the case on a consideration of the facts stated above. 
If, now, we attempt to reason back from effects to causes, 
we seem to be perfectly justified in concluding that any 
extensive tract of strata which are coloured, originally, by 
ferric oxide, must have been formed in inland lakes, under 
conditions of aridity, if not actually under desert conditions. 
We must be quite prepared, in studying such rocks in the 
field, to find evidence of changes in climatal conditions as we 
trace the strata upwards in the order of deposition. A red 
stratum, indicating arid conditions during its formation, 
may overlie or may be succeeded by another stratum formed 
under climatal conditions of quite a different kind. Such 
beds as these, coloured usually with ferric hydrate, and 
therefore yellowish or brownish to begin with, may be 
affected afterwards by later infiltrations, which may con- 
siderably modify the original colouring. Mr George Maw, 
of Broseley, has given us much interesting information 
upon this head. A stratum, red from causes acting at 
the time of its deposition, can usually be distinguished 
with ease from one in which the colouring is of secondary 
origin. 
These facts warrant us in coming to the conclusion that, 
under whatever conditions the Lanarkian Rocks, as a whole, 
were deposited, their red sandstones mark the result of desert 
conditions. The brown and grey beds of the Caledonian Old 
ted mark a change in climatal conditions. Like its older 
analogue, it was formed in inland lakes; but the wide 
diffusion of ferric hydrate, and the occasional occurrence 
of grey bands, show that the earlier part of the period was 
one during which the conditions ranged from sub-arid to 
sub-humid. On the platform where the grey, fossiliferous 
