"Vol. 52.] BASALT-PLATEAUX OF XORTH- WESTERN EUROPE. 343 



every feature of their arrangement can be satisfactorily determined. 

 In the memoir already cited I have described a number of examples 

 from the interior. I now proceed to give an account of other 

 instances from the coast-sections. 



I will begin with a group of vents in the Faroe Islands which 

 display with singular clearness some of the most characteristic 

 features of this part of the volcanic record. And here let me remark 

 that, although these islands have been so frequently visited and so 

 often described that their general structure is sufficiently well known, 

 they present in their details so vast a mass of new material for the 

 illustration of volcanic action that they deserve a far more minute 

 and patient survey than they have yet received. They cannot be 

 adequately mapped and understood by the traveller who merely 

 sails round them. They must be laboriously explored, island by 

 island and cliff by cliff. While I cannot pretend to more than a 

 mere general acquaintance with their structure, I have learnt by 

 experience that one may sail near their precipices and yet miss 

 some essential features of their volcanic structure. Last year I 

 passed close to the noble range of precipices on the western side of 

 Stromo, at the mouth of the Yaagofjord, and sketched the sill which 

 forms so striking a part of the geology of that district (figs. 24 & 

 25, pp. 380, 381). But I failed to observe an even more remarkable 

 and interesting feature at the base of the same sea-cliffs. This last 

 summer, probably under better conditions of light, I was fortunate 

 enough to detect with my field-glass, from the deck of the yacht, 

 what looked like a mass of agglomerate. Steaming inshore I was 

 delighted to find, as the vessel drew nearer to the cliff, that the 

 agglomerate assumed definite boundaries and occurred in several 

 distinct patches, until at last it presented the unmistakable out- 

 lines of a group of vents underlying and overspread by the bedded 

 basalts of the plateau. I at once got into the longboat, and, 

 favoured by an unusually calm sea, was enabled to steer into every 

 nook and round every buttress and islet of this part of the coast-line. 



The basalt-plateau here presents to the western ocean a nearly 

 vertical escarpment which must reach a height of at least 1000 feet 

 (see fig. 24, p. 380), and displays a magnificent section of the bedded 

 lavas. The lower part of this section shows chiefly the banded 

 structure already described, the layers of different consistency being 

 etched out by the weather in such a way as to give them the look of 

 stratified rocks. In the upper part of the precipice columnar and 

 jointed or prismatic sheets are more common, but the most pro- 

 minent band is the great sill just alluded to, and to which further 

 reference will be made in the sequel. 



In the course of the gradual retreat of the cliff, as the waves 

 tunnel its base and slice after slice is detached from its vertical 

 front, a group of at least five small vents has been uncovered lying 

 along a nearly north-and-south line. Of two of these a segment 

 remains still on the cliff-wall and passes under the basalts ; the 

 others have been dissected and half Cut away from the cliff, while 

 groups of stacks and rocky islets of agglomerate may mark the 



