396 PROFESSOR GEIKIE ON THE 
an ichthyolite, which was regarded by Professor HuxLry as resembling the 
Glyptolemus of the Upper Old Red Sandstone of Dura Den. 
There is, of course, no absolute proof that the Portskerry beds form a con- 
tinuous portion of the flagstone group. It might be contended, especially in 
view of the resemblance of the fish to an Upper Old Red Sandstone form, that 
the pale sandstones last described belong really to the same part of the system 


























































































































































































































































































































Fig. 5.—Unconformable Junction of Old Red Sandstone on 
Crystalline Rocks, Portskerry. 
as those displayed at Dunnet Head and Hoy. But, on the other hand, it should 
be observed that the sandstones perfectly resemble some of the yellow sand- 
stones of Reay and Sandside, which have already been described as clearly 
interposed among the flagstones; that from the dip of the flagstones on the east 
side of Bighouse Bay they must, unless faulted, overlie the beds at Portskerry ; 
and that their increasing sandy character towards their base plainly points to 
an approach to the crystalline platform. The sandy and brecciated strata at 
Portskerry are exactly what we should expect to meet with in the westward 
prolongation of the flagstones. It would never indeed occur to any observer 
examining the coast-line to separate them from the rest of the Old Red Sand- 
stone around them. a . 
To the west of Portskerry, for an interval of more than a mile, the cliffs 
consist of granite and a gnarled granitic gneiss, deeply trenched by narrow sea, 
inlets or goes, and fringed here and there with skerries and stacks. These 
rocks run out into the promontory of Rudha na Cloiche. But nearly on the 
furthest point of that headland a patch of grey sandstone, only a few square 
yards in extent, lies perched, with a gentle seaward dip, to indicate. the west- 

