John 0 Groat’s Horizon of the Old Red Sandstone. 257 
sandstones and shales, abundantly ripple-marked and sun- 
cracked, which form a gentle syncline, and crop out again on 
the east side of the bay, beyond which point the dip rolls 
over again to the east ; and a series of red and brown sand- 
stones, with frequent yellow sandstones, red shales, and thin 
bedded grey flags, forms the coast up to the Point of Ayre. 
The whole thickness of this series, from Dingyshowe to the 
Point of Ayre, a distance of three and a half miles, is about 
1200 feet. 
Although quarries are few, owing to the unsuitable nature 
of the rocks for building or road-making purposes, I have no 
doubt that the succession, as traced in the shore, holds good 
for the inland structure of the district; for, traced along the 
strike in Deer Sound, there is no evidence of any important 
dislocation; and again, the beds are cut by two massive trap 
dykes, one already mentioned running north and south from 
the Mermaid’s Castle, the other emerging at the Point of 
Ayre, and running north-west and south-east. Both of these 
I traced in several places inland, and found that they con- 
tinued in straight lines—one actually passing within a short 
distance of the quarry in which the fossils were found. 
Apart altogether from their characteristic fossil contents, 
the lithological character of the rocks of this horizon is 
worthy of notice. They distinctly resemble the Huna beds 
of the Caithness coast, and I have little doubt subsequent 
examination will show that they form a considerable part of 
the East Mainland of Orkney, as rocks having these charac- 
teristics, and containing D. macropterus in abundance, occur 
in several distinct localities in the district. On the other 
hand, in the district west of the great fault, which runs from 
Howquay Head to Kirkwall Bay, rocks of this facies do not 
occur, and from these beds none of the characteristic John 
o Groat’s fishes have been recorded. 
