520 Proceedings of the Royal Physical Society. 
these strata. are prevalently of a dark grey colour, which 
is due to the abundant diffusion of bituminoid matter. 
Bands tinted with ferric hydrate occur here and there, and 
there are also thin laminz containing more or less carbonate 
of lime or else dolomite. Bands of calcareous nodules of 
concretionary origin also abound on certain horizons. With 
these various constituents of the rock occur remains of fish, 
and also traces of vegetation, sometimes converted into coaly 
matter. 
Above the Orcadian Old Red lies unconformably the Upper 
Old Red, which, in one area or another, oversteps every rock 
older than itself. This rock again, though not red through- 
out, shows some red coloration, which is due, as in the 
other cases mentioned, to the presence of ferric oxide, which 
oceurs as films coating the grains of sand composing the 
main part of the rock. | | 
The whole of the rocks formed during the Devonian 
Period in Scotland of which remains 7 sitw still exist, 
appear to me therefore to have been formed under continental 
conditions and in inland lakes. At the very most there may 
have been once or twice local subsidences of sufficient 
amount to admit of communication with the sea. But of 
even this it appears to me that there does not exist any 
unequivocal evidence. This view is, of course, the same 
as that advocated by Godwin Austen, and afterwards so ably 
supported by Sir Andrew Ramsay. This latter author con- 
sidered that the Old Reds were formed under geographical 
conditions analogous to those now obtaining in the Aralo- 
Caspian area. 
There is no need on the present occasion to go into full 
detail regarding the physical geography of the area of inland 
drainage referred to, as this has often been treated of by 
others. There are, however, one or two points connected 
with this part of the subject which are of importance to take 
into consideration in an inquiry lke the present. One of 
these is that, under conditions of inland drainage, the whole 
of the water carried down by rivers is partly absorbed by the 
earth and the rest dissipated by evaporation. An important 
result, so far as the present subject is concerned, is that the 
