lOO PEOCEEDIIS^GS OF THE GEOLOGICAL SOCIETY. 



The topographical changes which preceded its deposition must have 

 involved no inconsiderable amount of subterranean disturbance, yet 

 the volcanic energy which had died out so completely long before 

 the close of the time of the Lower Old Eed Sandstone does not 

 appear to have been rekindled unfcil the beginning of the Carboni- 

 ferous period. 



Only one district in the British Isles, so far as I am aware, 

 furnishes any record of volcanic activity in the later part of the Old 

 Red Sandstone period.^ It lies far to the north among the Orkney 

 Islands, near the centre of the scattered outliers which I have 

 united as parts of the deposits of ' Lake Orcadie.' The thick 

 group of yellow and red sandstones which form most of the high 

 island of Hoy, and which, there can be little doubt, are correctly 

 referred to the Upper Old Hed Sandstone, rest with a marked un- 

 conformability on the edges of the Caithness flagstones. At the 

 base of these pale sandstones, and regularly interstratified with 

 them, lies a band of lavas and tufis which can be traced from the 

 base of the rounded hills to the edge of the cliff's, along the face of 

 which it runs as a conspicuous feature, gradually sloping to a lower 

 level, till it reaches the sea under the well-known sea-stack of the 

 Old Man of Hoy. It is about 200 feet thick, and consists of three 

 or more sheets of porphyrite, usually well marked off from each 

 other as separate flows. Beneath it and lying across the edges of 

 the flagstones below, there is a zone of duU red, fine-grained tuff, 

 banded with seams of hard red and yellow sandstone. 



On the low ground immediately to the north the flagstones are 

 pierced by several volcanic necks which we may with little hesita- 

 tion recognize as marking the sites of the vents from which this 

 series of lavas and tuffs was discharged. The largest of them forms 

 a conspicuous hill about 450 feet high, the smallest is only a few 

 yards in diameter and rises from the surface of a flagstone ridge. 

 They are filled with a coarse, dull green, volcanic agglomerate, made 

 up of fragments of the lavas with pieces of hardened yellow sand- 

 stone and flagstone. Around the chief vent the flagstones through 

 which it has been opened have been greatly hardened and blistered. 

 On the northern coast of Caithness I have described a remarkable 



^ There may be some examples of Upper Old Eed Sandstone volcanic rocks in 

 Ireland which I have not yet been able personally to examine. On the maps of 

 the Geological Survey (Sheet 198, and Explanation, pp. 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 the Kenmare estuary. 



