OLD RED SANDSTONE OF WESTERN EUROPE, 447 
‘flagstones pass westwards into sandstones and conglomerates as they approach 
the old shore-line at Reay (anie, p. 373). In like manner, as it seems to me, 
the flagstones of the upper Caithness groups pass southwards into the con- 
glomerates and alternating sandstones and clays of the great recess in Lake 
Orcadie, which is now chiefly occupied by the different northern firths. That 
the strata south of the Ord of Caithness are equivalents of some of the higher 
rather than of the lower parts of the Caithness flagstones is, I think, probable 
from the following considerations :— 
1. On the north side of the Ord of Caithness there is abundant evidence of 
an ancient shore-line ; but though conglomerates and sandstones occur along 
that line, they can be distinguished from those of the Moray Firth district in 
particular by the absence of those nodule-bearing clays, dark shales, and lime- 
stones which form so characteristic a feature throughout the latter district. 
Had the coast-line of Lake Orcadie at the beginning of the flagstone series run 
continuously from Aberdeenshire round the present margin of the Old Red 
Sandstone up to the Pentland Firth, we should have expected that the series of 
clays and nodules, which, with apparently every variety of shore,—bays, cliffs, 
inlets, gravel, sand, and mud,—yet continued to be formed along the whole of 
that extensive coast-line, would have stretched into Caithness also ; at least we 
could hardly be prepared for their sudden cessation. 
2. By regarding the conglomerates, sandstones, and clays south of the Ord as 
parts of the higher division of the Caithness flagstones, we overcome the 
difficulty which would otherwise arise, to account for the position of the Upper 
Old Red Sandstone along the southern shores of the Moray Firth. On this 
supposition the latter formation occupies the same relative place there as it 
does in Caithness and Orkney; that is, it rests unconformably on various 
members of the higher groups of the flagstone series. But if we make the 
underlying conglomerates and sandstones equivalents of the lower part of the 
Caithness series, we have to account for the want of any of the higher groups, 
and for the enormous denudation which must have intervened in that case 
previous to the time of the Upper Old Red Sandstone. There was undoubtedly 
some disturbance as well as denudation during the interval between the two 
formations, but the still gentle inclination of the strata all round the Moray 
Firth seems to indicate no very serious displacement of the older rocks. 
3. The strata south of the Ord agree more in petrographical characters with 
the higher than with the lower parts of the Caithness series. They much 
resemble some portions of the section about Huna and John o’ Groat’s, where 
alternations of red sandstone, grey shales, and clays, and dark limestones occur. 
The peculiar fish-bearing nodules of the Moray Firth I have not detected any- 
where in Caithness, but nodular shales-occur at intervals, even as far west as 
Balligil, in Sutherlandshire. 
