Yol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 339 



quarter, now towards another, yet everywhere retaining the universal 

 regularity and gentle inclinations of the whole volcanic series. 



These bare rocky fronts, while permitting us to observe the want 

 of continuity in many of the basalts, likewise afford an opportunity 

 of following any particular sheet over the whole of its outcrop. I 

 was particularly struck by the persistence of a dark band of basalt 

 in the lower part of the western declivity of Kuno. This sheet 

 can be kept in sight along the whole length of the island, at least 

 from a point opposite to Mygledahl in Kalso, with the exception of 

 a short concealed space of detritus at the mouth of the recess behind 

 the village of Kuno. It may possibly be even prolonged into the 

 island of Boro, for a similar band is seen occupying the same 

 position there. Its length on Kuno must be at least 6 nautical miles. 



The more the basalt-plateaux of Britain and the Faroe Islands 

 are studied, the more certain does the conclusion become that these 

 widespread sheets of lava never flowed from a few large central 

 volcanoes of the type of Etna or Vesuvius, but were emitted from 

 innumerable minor vents or from open fissures. In a later part of 

 this paper a number of the vents, which may still be seen under 

 the overlying sheets of basalt, will be described, and I shall point 

 out their resemblance to modern volcanic vents on the great lava- 

 fields of Iceland. 



In looking at one of the sea-cliffs of the Inner Hebrides or of 

 the Faroes, and in following with the eye its successive sheets of 

 lava in orderly sequence of level bands from the breaking waves at 

 the base to the beetling crest above, we are apt to take note only 

 of the proofs of regularity and repetition in the outflows of molten 

 rock and to miss the evidence that these outflows did not always 

 rapidly follow each other, but were separated by intervals of vary- 

 ing, sometimes even of long, duration. The layer of red bole or 

 decomposed lava, so often observable between the flows, has long 

 been regarded as evidence of the lapse of an interval sufficiently 

 extended to permit of considerable subaerial decay of the surface of 

 a lava-sheet before the outflow of the next lava. But an attentive 

 study of the plateaux discloses other and even more remarkable indi- 

 cations that the pauses between the consecutive basalt-beds were 

 frequently so prolonged as to allow of extensive topographical 

 changes being made in a district. 



The occurrence, for example, of interstratifications of different 

 kinds of sedimentary material among the lavas sufficiently demon- 

 strates the reality of these intervals of quiescence. Where this 

 material consists merely of volcanic tuff, it may only point to a 

 continuance of volcanic activity in the form of fragmentary dis- 

 charges during pauses in the outpourings of molten rock. In general, 

 however, there is not only tuff but non-volcanic sediment arranged 

 in definite layers, that show the action of running water. The 

 various clays (bauxite, lithomarge, etc.) and ironstones which lie 

 between the basalts of Antrim, besides their geological interest, 

 have now considerable economic importance. The clays, in par- 

 ticular, are much in request as sources for the manufacture of 

 aluminium. Neither among the Western Isles of Scotland nor in 



Q. J. G. S. No. 206. 2 a 



