OLD RED SANDSTONE OF WESTERN EUROPE. 411 
glomerate and the nearest part of the base of the Upper Old Red Sandstone (see 
fig. 8). This approach of the latter formation affords good ground for believing 
that we must here be very far removed from the true base of the flagstones. 
The deposits of Lake Orcadie have on the whole suffered little disturbance. At 
the time of the Upper Old Red Sandstone they must have been considerably less 
deranged than they are now. That formation therefore could hardly have been 
deposited on or near to the upturned and denuded base of the flagstones. . The 
Old Man. Wart Hill. ; Gremsa. 


Fig. 8.—Section from Gremsa to Old Man of Hoy, Orkney. 1. Granitic and schistose rocks. 2. Flagstones with 
conglomerate base. 3. Volcanic ‘‘neck.” 4. Lavas and tuffs forming base of Upper Old Red Sandstone, and 
lying unconformably on flagstones. 5. Yellow and red Upper Old Red Sandstones. 
importance of fixing as definitely as possible the position of these strata in the 
south-west of the Mainland arises from the fact that this particular district is 
one of the chief localities for the fossils of the Orkney Islands. 
3. Fossils—Many of the flagstones of Orkney are charged with organic 
remains. Especially is this the case with some of the dark, hard, fissile, 
| bituminous bands. On the surfaces of these strata remains of the characteristic 
| ichthyolites are crowded thickly together, and usually in such a tolerable state 
| of preservation as to show that the fishes must have died where their remains 
are found, or at least that they could not have been subjected to any prolonged 
| exposure and transport before they were buried under the accumulating 
sediment. As a rule, the fossils have been converted into a brittle jet-like 
substance, which isso liable to crack and ‘scale off, that unless great precau- 
tions are taken, an organism, which at first showed external sculpture in great 
beauty, becomes eventually a mere black. bituminous patch, retaining only 
the outline of the original specimen. It is not difficult, in most cases, to 
distinguish an Orkney from a Caithness ichthyolite. ! 
The fossil plants of Orkney include most of the forms found in the Thurso 
\and upper Caithness groups. | 
Crustacea are represented in the Orkney flagstones by two forms, according 
to present knowledge. The little Estheria membranacea, so abundant at 
Thurso, was first observed by Sir R. Murcuison and Mr C. W. Peacu to be 
equally plentiful in some of the flagstones at Kirkwall,* thus corroborating by 

* “Quart. Journ. Geol. Soc.” xv. p. 404. 
