OLD RED SANDSTONE VOLCANIC EOCKS OF SHETLAND. 361 



I. Geological Structure of the Volcanic Rocks. 



The records of volcanic activity are mainly confined to the west and north- 

 west portions of the Mainland and the islands adjoining the western seaboard. 

 They may be grouped in two divisions, — first, the contemporaneous igneous 

 rocks, comprising the lavas and tuffs which were erupted and spread over the 

 sea-floor during the accumulation of the sedimentary deposits ; second, the in- 

 trusive igneous rocks, which were injected at a later date, probably towards 

 the close of the Old Red Sandstone period in Shetland. The result of the 

 chemical analysis of typical examples of these divisions clearly proves that 

 the former belongs wholly to the basic series, while the latter includes both 

 acidic and basic rocks. 



A. Contemporaneous Lavas and Tuffs. 



Beginning first with the interbedded volcanic rocks, the best development 

 of them is to be found in the south-west part of Northmavine, between Stenness 

 and the mouth of Rooeness Voe. No finer sections could be desired than 

 those exposed along the storm-swept cliffs of the Grind of the Navir. Here 

 and there the observer sees narrow "gios " which have been excavated in the 

 tough lavas and ashes and occasionally a subterranean passage or tunnel, com- 

 municating with the surface by a funnel-shaped aperture, from which, during 

 storms, a column of spray issues with the advancing tide. The best examples 

 of this latter phenomenon are to be seen at the " Holes of Scraada." 



The tract of ground occupied with this series of ancient lavas and tuffs 

 measures about six miles in length from Stenness to Ockren Head. It is evi- 

 dent, however, that they must originally have covered a larger area, from the 

 isolated fragments which have escaped denudation, in the islet of Doorholm, 

 and Esha Ness Skerry. In the southern part of this tract, between Stenness 

 and Hamna Voe, the terrace-shaped features which are characteristic of vol- 

 canic areas are so apparent, that the eye can easily follow the successive out- 

 crops of the lavas and tuffs. This area is almost entirely occupied with 

 contemporaneous volcanic rocks, there being but few intercalations of sedimen- 

 tary deposits. Along the east side, between Rooeness Voe and Brei Wick, the 

 lavas and tuffs are bounded by a great sheet of granite and quartz -felsite, which 

 will be described in a subsequent page. On the south bank of Rooeness Voe, 

 rather more than a mile from Ockren Head, the relation between the two is 

 admirably exposed in a steep grassy " gyo." At this locality the slaggy por- 

 phyrites, which form a cliff about 300 feet high, with a beautifully slicken-sicled 

 surface, are brought into conjunction with the pink granitoid rock by a fault. 

 Owing to the covering of peat, we were unable to trace this fault across the 



