35° 
VOLCANOES OF THE UPPER OLD RED SANDSTONE 
BOOK V 
ciiicl SciiiclstoiiGS undcrlio tliis tuff ciiicl crop out between it (iiid the uggloiu- 
erate. There is tlius evidence of tlie intercalation of volcanic tuff in tlie 
Upper Old Red Sandstone of this district. And there seems no reason to 
doubt that the tuff was ejected from the adjoining vent of Knockfeerina. 
On the next ridge of Old Red Sandstone, which runs parallel to that of 
Knockfeerina at a distance of little more than a mile to the south, another 
mass of volcanic material rises into a prominence at Ballinleeny. On the 
north side it consists of agglomerate like that just described, and is 
flanked by sandstone baked into quartzite. Here again we probably see 
the edge of a volcanic funnel. There may possibly be more than one vent 
in this area. But well-bedded tuffs, can be observed to dip aw'ay from 
the centre and to pass under sandstones and shales which are full of 
fine ashy material. Oradations can be traced from the tuff into ordinary 
sediment. In this instance, therefore, there is additional proof of contem- 
poraneous volcanic action in the Upper Old Red Sandstone. There can be 
no uncertainty as to the horizon of the strata in which these records have 
been pieserved, for they dip conformably under the shales and limestones 
at the base of the Carboniferous Limestone series. They are said to have 
yielded the characteristic fern Palwopteris of Kiltorcan.^ 
THE NORTH OF SCOTLAND 
The only district in England or Scotland wherein traces of volcanic 
action during the time of the Upper Old Red Sandstone have been ob- 
served lies far to the north among the Orkney Islands, near the centre of 
the scattered outliers which I have united as parts of tlie deposits of “ Lake 
Orcadie ” ^ (Map I.). The thick group of yellow and red sandstones which 
form most of the high island of Hoy, and whicli, there can be little doubt, 
are correctly referred to tlie Upper Old Red Sandstone, rest with a marked 
unconformability on the edges of the Caitliness flagstones (Fig. 103). At 
the base of tliese pale sandstones, and regularly interstratified with them, 
lies a band of lavas and tuffs which can be traced from the base of the 
rounded hills to the edge of the cliffs at 'the Cam, along the face of which 
it runs as a conspicuous feature, gradually sloping to a lower level, till it 
reaches the sea. At the Cam of Hoy it is about 200 feet thick, and con- 
sists of three or more sheets of andesite. The. upper 50 or 60 feet show a 
strongly slaggy structure, the central portion is rudely columnar, and the 
lower part presents a kind of horizontal jointing or bedding. There can be 
no question that this rock is not a sill but a group of contemporaneous lava- 
flows. Beneath it, and lying across the edges of the flagstones below, there 
) Tliore may be some other cxamirles of Upper Old Red Sandstone volcanic rocks in Ireland 
whicli I have not yet been able personally to e.xamine. On the maps of the Geological Survey 
(Sheet 198, and Explanation, jip. 8, 17) contemporaneous rocks of this age are marked as 
occurring at Cod’-s Head and Dursey Island, on the south side of the mouth of tlie Kenmare 
estuary. 
iir.st noticed in Gcol. May. February 1878; and Tmns. Hoy. Soc. Edin. xxviii. (1878), p. 411. 
