446 PROFESSOR GEIKIE ON THE 
sandstones, which extend to Tarbat Ness, and have been referred to the Upper 
Old Red Sandstone. In examining this coast section I could not satisfy my- 
self of the existence of any unconformability between the two sets of rocks, 
But of course it might very well exist without being traceable in a few yards of 
beach. Some faulting occurs at the junction, which rather obscures the relations 
of the rocks. 
Horizon of the Lower Old Red Sandstone in the Basin of 
the Northern Firths. 
We have now traced the deposits along the remaining coast-line of the 
ancient Lake Orcadie from Banffshire round to Sutherlandshire. We have 
found them throughout that extended tract mainly conglomeratic and arenaceous 
—the conglomerates in thick masses coming in again and again on successive 
platforms, while interstratified with them lie bands of grey clay and shale full 
of calcareous nodules containing fish remains. These fossiliferous bands retain 
their distinctive characters, lithological and paleontographical, throughout the 
whole district. In no part of this Old Red Sandstone belt does any great 
thickness of strata appear, the belt being itself narrow and the usual angles of 
inclination low. There can be little hesitation in regarding the whole of this 
tract as part of one continuous area of deposit, and in recognising in its strata 
the deposits of an old shore and of the shallow water near land. 
When we trace these deposits almost up to the verge of Caithness, and, 
crossing into that country, come upon the vast continuous flagstone series, 
we cannot but be struck by the remarkably rapid change in the character 
of the strata. The hills of Sutherlandshire, ending in the granitic pro- 
jection of the Ord of Caithness, separate two very distinct groups of | 
rocks. In tryimg to parallel these groups, we naturally begin by assuming | 
that the basement conglomerates on the one side represent in strati- | 
graphical position, and generally in time the corresponding strata on the | 
other side. The reddish-grey sandstones, blue-grey nodule-bearing clays, 
and rapid alternations of grey and dark bituminous flagstone, shale, and thin | 
limestone, which succeed the conglomerates in Easter Ross, do not, however, at 
all accord with the flaggy sandstones and enormously thick flagstones of | 
Caithness. It is hardly possible that the comparatively trifling thickness of | 
strata in the Moray Firth can really be the true equivalent of the vast Caithness | 
series. The attenuation of so massive a succession of deposits would be too 
rapid. I believe the true explanation to be that the Old Red Sandstone south 
of the barrier of the Ord of Caithness represents only the upper part of the 
Caithness flagstone series. We have already seen on what an uneven surface 
that series was laid down, and likewise how unequal was the movement of| 
depression during which the deposition took place. The Thurso and Forss 

