OLD RED SANDSTONE OF WESTERN EUROPE. 379 
of Brough, but without reaching so high as the zone of red sandstone seen to 
the east of Mey. They do not therefore require to be added to the total thick- 
ness above given for the Caithness flagstones. 
If now, by way of comparison, we trace a section from the base of the Old 
Red Sandstone about Loch Shurrery across the prevalent strike of the strata to 
Holburn Head,—a distance of 10 miles ; and if we assume the average general 
angle of dip to be 10°, which is probably very near the truth, we obtain a thick- 
ness of strata along that line amounting to nearly 9000 feet. To that thickness 
would require, of course, to be added the 800 or 900 feet of flagstones over- 
lying the Holburn Head beds to the west, and likewise the still higher portions 
of the Thurso flagstone series, as well as the depth of the red sandstone and 
flagstone series of Mey, Huna, and John o’ Groat’s, amounting to about 3400 
feet. These sums would make a total probably not very widely different from 
the measurement already given from actual observations on the coast-line. We 
may regard the Old Red Sandstone in Caithness, therefore, as probably attain- 
ing a thickness of more than 15,000 feet. 
Before quitting this part of the subject, a remarkable feature may be 
noticed here, to which fuller reference will afterwards be made. On glancing 
at the arrangement of the rocks, as shown by the dip arrows on the map 
(fig. 4), we observe that in the north-western part of the county the flagstones 
for some miles strike persistently at the crystalline rocks of the underlying 
platform. It might be supposed that this structure is due here, as it is com- 
monly, to the influence of a fault, by which the older and younger rocks have been 
brought abruptly against each other along a sharp and rectilinear boundary-line. 
Such, however, is not the case. From a point on the coast about a mile to the 
west of Sandside Head, the gneiss, granite, and other crystalline masses may be 
followed south-eastwards for five or six miles, with the conglomeratic and sandy 
ends of the flagstone strata resting upon them ; but, instead of dipping away 
from them, retaining still their prevalent north-westerly inclination. Hence the 
strata which eastwards from Sandside Bay occur as ordinary calcareous fissile 
flagstones, pass along their line of strike into sandstones and conglomerates as 
they abut upon the fundamental platform. A conglomeratic base at any point 
does not therefore necessarily mark the actual bottom of the whole formation. 
On the contrary, the conglomerates of Reay belong to a high zone in the flag- 
Stone series. But they are of course merely local, and disappear as soon as the 
Strata begin to recede from the older rocks. We perceive also how importantly 
this structure bears upon the history of the Caithness flagstone series, since it 
proves the gradual depression of their area of deposit, and enables us in some 
measure to trace the outline of that area and the direction of its greatest 
depression. Similar evidence is furnished in Orkney as well as along the basin 
of the Moray Firth. 
VOL, XXVIII. PART IL. 5 E 
