Lituminoid Cement of the Caithness Hlagstones. 327 
of lime, which on its passage to the surface passes through 
beds of peat, is quite sufficient to account for the phenomena 
in question. The very common association of deposits of 
sulphur with those of these bituminoids points also to the 
same conclusion, as this would represent one of the ultimate 
products of decomposition of the sulphate of lime. 
Given conditions of inland drainage where the nature 
of the rainfall within the drainage area permits of the 
growth of vegetable matter, it seems to me that all the 
rest follows as a necessary consequence. ‘The sediments 
formed under such conditions would contain in a diffused 
form —(1) limonite (ferric hydrate); (2) calcareous or 
dolomitic matter; (3) bituminoids in quantity proportionate 
to that of the vegetation carried into the lake. They may 
contain in addition deposits of rock-salt and gypsum, together 
with any of the “abraum” salts commonly associated with 
these. But they will contain no iron in the form of ferric 
oxide. 
I should therefore regard the red parts of the Old Red 
(whether in the Orcadians or other) as having been deposited 
in areas of inland drainage at a time when the climatal 
conditions here were in the main too arid to permit of the 
growth of almost any vegetation. On the other hand, such 
erey, bituminiferous beds as the plant-bearing beds of the 
Caledonian Old Red, and also the Caithness Flagstones 
proper, represent other parts of the Devonian Period when 
conditions of inland drainage still prevailed, but when the 
climate for the time being approached the normal, or humid 
type, and when, in consequence, vegetation flourished. 
It is to the action of weak solutions of sulphate of lime 
upon the vascular tissues of such of this vegetable matter as 
was swept into the old inland lakes, that I am disposed to 
attribute the formation of the bituminoid cement of the 
Caithness Flags. 
