OLD RED SANDSTONE OF WESTERN EUROPE. 409 
ficulty in making a tabular arrangement of the rocks in almost any one of 
the islands. 
The flagstones retain the same features so well-marked in Caithness. Some- 
times, as at Skaill in Pomona, they are exceedingly hard, fissile, bituminous, 
and crowded with fossil fish. In other places, as near Kirkwall, they form 
thicker beds, and can be quarried for building materials, like those which are 
similarly used at Thurso. Bands of dull red and even yellowish sandstone 
occur interstratified with the flagstones, as in Scapa Bay, Meal near Kirkwall, 
Eday, and other places. Where the flagstones rest on the old crystalline rocks 
they become for a short space conglomeratic at the base. A conglomerate, as 
T am informed by my friend Professor HEppLe of St Andrews, occurs at 
Heglabir, on the west side of Sanday, upon a band of red sandstone which 
passes down into the flagstone series. Sir R. Murcuison thought it probable 
that the light yellow and whitish sandstones of Eda and Shapinsha belong to 
the Upper Old Red Sandstone.* I did not land on these islands, but passing 
close to their shores, I saw nothing which did not seem to me a conformable 
part of the flagstone series. Dr HEpDDLE, at my request, examined this question 
on the ground, and confirms my inference. Dark flagstones always rise from 
under these sandstone bands, which therefore cannot be identified with the 
great lower red sandstone masses of Caithness. Indeed there does not appear 
to be in Orkney any trace of these lower parts of the system. (See Plate XXII, 
column ii.) 
2. Position and Order of Succession among the Strata—As the flagstones of 
the Orkney Islands have supplied a considerable proportion of the fossil fishes 
of the Scottish Old Red Sandstone, it becomes of importance to define as 
nearly as possible their place in the system. Fortunately, from their position 
with reference to the clear sections in the adjacent Caithness tract, this can be 
satisfactorily done. 
It has been already mentioned that the upper parts of the great Caithness 
Series stretch across the Pentland Firth and reappear in the southern of 
| 
the Orkney Islands. Stroma is formed merely of a prolongation of the 
Huna group of flagstones. In South Walls, Fara, Flota, and South 
Ronaldsha similar flagstones extend mile after mile along the picturesque 
cliffs of these flat-topped and otherwise featureless islands. Here and there, 
as in the north-west of South Ronaldsha, occur the zones of red sandstone 
above referred to, which may be parallelled with the John o’ Groat’s 
and Gill’s Bay groups of Caithness. As we advance northwards among 
the islands the same petrographical characters continue. The dip of the strata 
* Op. cit. p. 410. 
.VOL, XXVIII. PART II. es 50 
