CHAP. XXI ERUPTIONS IN THE ORKNEY AND SHETLAND ISLES 345 
aniygdaloidal and vesicular iipper surfaces. Their thickness cannot he 
ascertained, as their base is not seen, but tliey have l^een cut by the 
sea into trenches which show them to exceed dO feet in depth. The 
position of the vent from which they came has not been ascertained. 
Neither here nor in the Moray Firth area do any sills accompany the 
interbedded sheets, and in both cases the volcanic action would seem to have 
been of a feeble and short-lived character. 
]\Iuch :nore important were the volcanoes that broke out nearly 
100 miles still further north, where the Mainland of Slietland now 
lies. I shall never forget the pleasure wdth which I first recognized the 
traces of these eruptions, and found near the most northerly limits of the 
British Isles proofs of volcanic activity in tlie Lower Old Bed Sandstone. 
Since my observations were published,' Mr. Peach, who accompanied me in 
Shetland, has returned to the district, and, in concert with his colleague 
Mr. Horne, has extended our knowledge of the subject.^ The chief vent or 
vents lay towards the west and north-west of the klainland and North 
Marine ; others of a less active and persistent type were blown out some 
25 miles to the east, where tlie islands of Bressay and Noss now 
stand. In tlie western district streams of slaggy andesite and diabase with 
sliowers of fine tuff and coarse agglomerate were ejected, until the total 
accumnlation reached a thickness of not less than 500 feet. The volcanic 
eruptions took place contemporaneously with the deposition of the red sand- 
stones, for the lavas and tuffs are Intercalated in these strata. The lavas 
and volcanic conglomerates are traceable from the southern coast of Papa 
Stour across St. Magnus’ Bay to the western headlands of Esha Ness, a 
distance of more than 14 miles. They have been cut by the Atlantic 
into a picturesque range of cliffs, which exhibit in some places, as at the 
singular sea-stalk of Iforeholm, rough banks of andesitic lava with the 
conglomerate deposited against and over them, and in other places, as 
along the cliffs of Esha Ness, sheets of lava overlying the conglomerates. 
No trace of any vents has been found in the western and chief volcanic 
district, but in Noss Sound a group of small necks occurs, filled with a coarse 
agglomerate composed of pieces of sandstone, flagstone and shale. Messrs. 
Peach and Horne infer that these little orifices never discharged any streams 
of lava. More probably they were opened by explosions which only gave 
forth vapours and fragmentary discharges, such as a band of tuff which is 
intercalated among the flagstones in their neighbourhood. 
But one of the most striking features of the volcanic phenomena of this 
remote region is the relative size and number fif the sills and dykes which 
here as elsewhere mark the latest phases of suliterranean activity. Messrs. 
Peach and Horne have shown us that three great sheets of acid rocks 
(granites and spherulitic felsites, to which reference has already been made, 
p. 292) have been injected among the sandstones and Ijasic lavas, that 
abundant veins of granite, (|uartz-felsite and rhyolite radiate from these 
acid sills, and that the latest phase of igneous action in this region was the 
1 Trans. Roy. Soc. Min. vol. xxviii. (1878), p. 418. ^ Ibid. vol. x.xxii. (1884), p. 359. 
