398 PROFESSOR GEIKIE ON THE 
regards the structure and history of the Old Red Sandstone. Unfortunately, 
owing partly to the obscurity arising from extensive accumulations of blown 
sand and of peat, partly to the existence of faults and frequent foldings of the 
rocks, it is not possible in the meantime positively to identify any of the beds 
now to be traced with those which have already been described. Judging from 
the evidence at present available, we may with some probability place the flag- 
stones of Brough, which lie at the western end of the section, not far from the 
position of those of Castletown. As the dip is mostly towards E.N.E,, there 
must be a repetition of the strata with a gradually ascending section towards 
the east. But in this case the line of coast cuts sharply across the strike of the 
beds, with the result of bringing us to much higher portions of the flagstones 
than can be seen to the west of Dunnet Head. 
Between Dunnet Bay and Brough the great promontory of Dunnet Head 
projects into the Pentland Firth, with its long lines of precipice barred by the 
gently inclined stratification of their yellow and red sandstones. These strata, 
so different from everything around them, have no doubt been correctly assigned 
by Murcuison to the Upper Old Red Sandstone. They will be again referred 

Fig. 6.—Section in Brough Bay. 1. Caithness Flagstones ; 2. Upper 
Yellow Sandstones ; F. Fault. 
to in a subsequent part of this memoir. On the west side of the promontory — 
no junction can be seen between these flat or very slightly sloping strata and 
the more highly inclined flagstones of Dunnet Bay. On the east side, however, 
the contact of the two series of rocks can be examined to great advantage in the 
Bay of Brough. Through the centre of that bay there runs a fault, by which the 
yellow sandstones on the west are thrown on end and contorted against the flag- 
stones on the east side. (See fig. 6.)* Sir Roperick Murcuison believed that the 
Caithness flags pass up conformably into these yellow sandstones, and referred - 
to the shore at Brough as one of the places where this passage could be made 
out. Apart, however, from the striking dislocation between the two formations, 
by which of course any chance of tracing a gradation between them must be 
* The existence and effect of this fault were noted by Sepewick and Murcuison, op. cit. p. 133. 
In his later memoir, on the “ Succession of the Older Rocks in the Northern Highlands,” Murcutson, as 
stated above, referred to the locality as one where the flagstones pass under the overlying yellow sand- 
stones, and he actually gives a section showing the conformable superposition of the latter strata upon 
the former. (“Quart. Journ. Geol. Soc.” xv. 409.) 

