OLD RED SANDSTONE OF WESTERN EUROPE. 403 
pointed out, intercalations of red strata, the precursors of the thick mass 
of Gills Bay, are found on the west side of the dislocation regularly inter- 
stratified in the upper part of the flagstone group. We have here, there- 
fore, in ascending succession a higher conformable series of red sand- 
stones. In the memoir by Sep@wick and Murcuison the red sandstones 
of the north of Caithness are spoken of as one series, sometimes as 
“the newer sandstone,’ sometimes as “the upper red sandstone.” Under 
these terms the sandstones of Dunnet Head, Gill’s Bay, and John o’ Groat’s 
were included as portions of one great set of deposits resting conform- 
ably upon, and passing down into, the flagstones. It will now be shown, how- 
ever, that there are in this part of Caithness three distinct zones of red sand- 
stone. Two of these belong to the higher part of the flagstone series ; while 
the third or Dunnet Head zone, which has with probability been assigned to the 
Upper Old Red Sandstone, as indicated in an earlier part of this essay, must 
lie unconformably upon the flagstones. 
On the west side of Gill’s Bay the shore exposes a series of ledges of red 
false-bedded sandstone, dipping gently eastward. Many of these strata greatly 
- resemble parts of the Dunnet Head or Upper Old Red Sandstone series, so that 
it is not in the least surprising that they should have been classed together. 
My first impression was that they were identical. Further examination, how- 
| ever, showed not only that the base of the sandstones was interstratified with 
_ the flagstones, but that their top in like manner was interleaved with seams of 
flagstone, and passed under a thick overlying and conformable flagstone group. 
The latter relation can be satisfactorily established along the southern shore of 
Gills Bay. Alternations of friable red sandstones with red and grey flagstones, 
after dipping eastward like the thick zone of sandstone underneath them, begin 
‘to undulate and then turn round so as to dip towards N.N.W. This change 
so nearly coincides with the trend of the coast-line that the reefs of hard red 
sandstone and flagstone (one of which is particularly prominent) run almost 
parallel with the beach as far as the Ness of Quoys, where a portion of the 
Gill’s Bay sandstone is brought up along a sharp anticline. To the east of that 
promontory there is a steadily ascending section for nearly three miles, the strata 
being always visible in shore reefs, and also for much of the distance in a line of 
low sea-cliff. The same alternations of red sandstones and grey flags occur on 
the east side of the Ness of Quoys, and these are soon succeeded by a thick 
| zone of grey flagstone, which extends along the shore below Huna. 
Making allowance for the undulations of dip, and taking the limits of the 
| Gil’s Bay sandstone zone to be defined by the cessation of the red sandstone 
| bands in the flagstones below and above, we may allow 400 feet for the thick- 
| ness of this group. No fossils have yet been found in these sandstones, so far 
as I have been able to learn. The group is seen nowhere else in Caithness ; 
| 
