410 PROFESSOR GEIKIE ON THE 
is perhaps on the whole towards W. or N.W. But it undulates usually in broad 
folds, though sometimes, as at the southern end of Eda, in sharper bends. But 
neither by the prevalence of a uniform dip, nor by any marked change in the — 
lithology, is the traveller led to notice any great ascending series of strata. He 
seems to be for ever meeting with repetitions of the same rocks. No doubt — 
when these islands come to be mapped in detail, the real thickness of flagstones 
will be found to be more considerable than might at first have been surmised. 
Although no equivalents are met with of the massive red sandstone, shales, 
and conglomerate groups at the base of the Caithness series, yet on the south- 
west side of the Mainland, the axis of old crystalline rocks above mentioned 
rises through the flagstone. It begins on the coast at Inganess, runs 8.8.E. 
to Stromness, and reappears in the middle of the island of Gremsa. In 
this axis granite, and a coarse gneiss passing into a foliated granite, and crossed 
by many veins and threads of pink felsite, come to the surface. Its north end 
is truncated by a small fault. But both on the east and west sides the over- 
lying flagstone series may be seen resting unconformably upon and _ partly 
derived from these ancient rocks. A few yards of brecciated conglomerate 
and sandstone form the base of the series. There is but little here to recall 
the base of the Old Red Sandstone of Caithness. Indeed, in walking over the 
sections the observer is constantly led to realise that he has here but a mere © 
local interruption of the flagstone series, due to the rise of an old ridge of rock 
from the surface of the sheet of water in which these strata were accumulated, 
and that the conglomerates are not the actual base of the formation. Passing 
southwards into Hoy and the islands lying to the south-east he finds that as 
the general average dip there is towards the north or the north-west, the strata 
overlying the crystalline ridge must really lie some considerable way up in the 
Orkney series. Here therefore is another evidence, in addition to those cited 
from Caithness, of the gradual subsidence of the very uneven surface of the Old ‘ 
Red Sandstone land beneath the waters of Lake Orcadie.* } 
In connection with the position of the Stromness and Gremsa conglomaaaaa 
it should be noticed that the Upper Old Red Sandstone of Hoy passes unconform- 
ably over a portion of the flagstone series not far above that conglomerate. 
A section drawn from the centre of Gremsa to the opposite hills of Hoy gives a 
thickness of probably not less than 2000 feet of flagstone between the con- 

* Sir R. Murcutson (“ Quart. Journ. Geol. Soc.” vol. xv. p. 410), in describing the conglomerate of 
Stromness, refers it without question to the same basement position as the lowest conglomerates of 
Caithness. He likewise speaks of the red sandstone of Kirkwall as occupying a similar low horizon. 
Further examination, however, would have shown that these sandstones not merely underlie, but are 
interstratified with and overlie flagstones. The coarse conglomerate of well-rounded sandstone blocks 
at Heglabir on the west side of Sanday, which has been long known (see Barry’s “ Orkney,’ p. 56, 
and Net’s “ Tour”), seems to occur at a greater distance from the local base, for it is said to overlie 
sandstones and flagstones. (See ante, p. 409.) 
