598 
Proceedings of the Pioyal Society 
three groups, Lower, Middle, and Upper, found in consecutive 
order; that this so-called “Middle” division occurs only in the 
north of Scotland, where it lies unconformably upon the older 
palaeozoic rocks, and is itself unconformably overlaid by the Upper 
Old Eed Sandstone, thus occupying a position exactly similar to 
that of the Lower Old Eed Sandstone on the southern side of the High- 
lands. He further points out that while some species of fishes are 
common to the Old Eed Sandstone on the two sides of the High- 
land barrier, the lithological differences between the deposits of the 
two areas are so great as to make it evident that the rocks were laid 
down in distinct basins, and consequently that the fauna of each 
basin, might be expected to be more or less peculiar, as in many 
analogous cases at the present day. As evidence that adjacent 
areas in the time of the Lower Old Eed Sandstone were strongly 
marked off from each other in their faunas, reference is made to 
the contrast between the fishes and crustaceans of the Welsh region 
and those of Lanarkshire and Eorfarshire, not a single species being 
common to the two countries, though some of the genera are. 
Eeasons are then given why the argument used by Murchison from 
the occurrence of many of the Scottish ichthyolites in Eussia could 
not be regarded as establishing the existence of a “ Middle ” divi- 
sion of the Old Eed Sandstone. 
The conclusion arrived at by the author is that the Caithness flags 
or “ Middle Old Eed Sandstone” are probably the general equiva- 
lents of the Lower Old Eed Sandstone of other regions, and that 
this system consists in Britain of two well-marked divisions only — a 
Lower, which graduates in some places into the Upper Silurian 
rocks, and is separated by an unconformability from an Upper, 
which in many districts passes up into the base of the Carboniferous 
system. 
The various districts into which the area embraced under the 
term Lake Orcadie may be divided are then described seriatim. 
The detailed structure of Caithness has been worked out by the 
author (partly with the co-operation of his colleagues in the Geological 
Survey, Mr B. H. Peach and Mr John Horne) as affording the most 
complete sections of the Old Eed Sandstone in the north of Scot- 
land. Arranged in descending order, the various stratigraphical 
zones stand as in the subjoined table : — 
