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OLD RED SANDSTONE OF WESTERN EUROPE. 399 
destroyed, it will be clear upon reflection that the flagstones of Brough cannot 
possibly graduate into the yellow sandstone. Instead of lying at the top of the 
Thurso flagstone group, they must be somewhere near its bottom ; separated, 
therefore, by more than 8000 feet of strata from the yellow sandstones, even if 
these reposed conformably upon the Caithness flagstone series. We have no 
reason to suppose the throw of the Brough fault to be very large; but even 
should it be considerable, it could not account for the position of the flat sand- 
stones of Dunnet, against which the more highly inclined flagstones, on the 
south-west side of Dunnet Bay, are striking, and under which they no doubt 
pass. Though an unconformability cannot be proved here by any actual section, 
that is the only relation of the two formations which will explain the geological 
structure of the ground. It will be afterwards shown how the unconfor- 
mability is actually demonstrated by clear sections on the opposite island of 
Hoy. 
Starting, then, from the fault in the middle of Brough Bay, let us trace the 
succession of beds eastwards. At first the flagstones undulate slightly ; they 
then assume an inclination to N.N.W., which soon veers round to W.N.W., the 
angle of dip rising rapidly from 15° to 40°. These strata exactly resemble parts 
of the section in Dunnet Bay. They continue in descending section as far as 
Ham, where an anticlinal axis occurs, beyond which the dip, with only 
occasional and trifling exceptions, remains easterly, or tending towards north- 
east. It is on this axis that the lowest strata along the south side of the Pent- 
land Firth are brought up. From that point on the coast an ascending series 
can be traced through the upper flagstones into higher strata than are elsewhere 
in Caithness seen in conformable sequence. Much greater facilities exist for 
the detailed examination of the rocks along this northern shore than in other 
parts of the coast-line of the county ; for, except at the cliffs of Brough, where 
the flagstones rise into mural precipices 100 feet in height, these strata form a 
sinuous line of low indented cliffs, easily accessible in many places, and fringed 
by shore ledges and reefs, where wide sheets of the strata can be walked 
upon. 
At Ham grey flagstones of the usual type form the crown of the anticlinal 
arch, dipping to north-west and north-east in broad sheets, which slope into the 
Water at angles of from 8° to 15°. Strata of the same character succeed in 
ascending order towards the east, the dip continuing north-easterly at angles 
ranging between 8° and 25°. Here and there small sharp folds occur, some- 
times accompanied by faults. In one case the line of an arch of the flagstones 
coincides with the trend of a picturesque goe; in another, at the haven of 
Skarfskerry, the strata, bent into a sharp fold, with crumpling of the shales, run 
out as a reef into the sea. In the western promontory of Skarfskerry thicker 
bedded flagstones appear, and among these there occurs a band of lumpy dull- 
