Bituminoid Cement of the Caithness Flagstones, 319 
instead of red, as those are in which the colouring matter 
consists of ferric oxide. To this division belongs the volcanic 
rocks of the Ochils, Sidlaws, Pentlands, etc. Succeeding 
these volcanic rocks in the areas to the north of the Forth 
is a band of grey flagstones in which occur fossil fish 
(Cephalaspis and its allies, and some of the Acanthodians) 
and frequent remains of fossil plants. Bituminoid matter is 
present in these grey beds containing the organic remains, 
and ferric oxide is conspicuous by its absence. These grey, 
bituminiferous, and plant-bearing beds are succeeded by the 
sandstones and marls of Strathmore, which are, prevalently, 
red, with the pale green decoloration marks which so often 
accompany strata coloured by ferric oxide. Fossils of any 
kind are rare or absent entirely from these red beds, and 
bituminoids are equally conspicuous by their absence. The 
total aggregate thickness of the beds belonging to the 
Caledonian division of the Old Red must be close upon, 
if it does not exceed, 20,000 feet. 
At the present day it is no longer possible to trace the 
connection between the highest or any other subdivision of 
the Caledonian Old Red and any part of the Old Red of the 
type occurring in the Moray Firth. These latter rocks 
present, in their lower portions especially, much the same 
petrographical characters as the Caledonian Old ed, 
Calcareous concretions, however, occur in the lower parts 
of these rocks, and these concretions have yielded numerous 
fish remains, which Dr Traquair regards as pertaining to a 
later stage of evolution than the fish remains from the 
Caledonian Old Red. In tracing the Old Red rocks of the 
Moray Firth northward, we find the members of the series 
there assuming a somewhat different type, which prevails 
throughout the greater part of Caithness, and extends 
northward into Orkney. The rocks of this type consist of 
flagstones, which, like the other sedimentary members of 
the Old Red, appear to have been formed either as torrential 
deposits on the land, or else as layers deposited under 
shallow-water conditions in inland lakes. But instead of 
being characterised by a coloration due (as the brown is) 
to ferric hydrate, or as the red coloration is, to ferric oxide, 
