July 15, 1897] 



NATURE 



243 



plateau-type had generally ceased, volcanic activity 

 showed itself over the area of the British Isles in a 

 different guise, both as regards the nature of its pro- 

 ducts and the manner and scale of their discharge 

 (vol. i. p. 414). Instead of widely extended lava-sheets 

 and tuffs, piled above each other sometimes to a thick- 

 ness of many hundred feet, and stretching over hundreds 

 of square miles, another phase of volcanism presented 

 itself, where scattered groups and rows oi Ptiys, or small 

 volcanic cones, threw out, in most instances, merely 

 tuffs, and these often only in trifling quantity, though 

 here and there their vents also poured forth lavas, and 

 gradually piled up volcanic ridges which, in a few cases, 

 almost rivalled some of the plateaux. The evidence for 

 these less vigorous manifestations of volcanic activity is 

 furnished (i) by layers of tuff and sheets of basaltic- 

 lavas intercalated among the strata that were being 

 deposited at the time of the eruptions ; (2) by necks of 

 tuff, agglomerate, or different lava-form rocks that mark 

 the positions of the orifices of discharge ; and (3) by 

 sills, bosses, and dykes that indicate the subterranean 

 efforts of the volcanoes. The comparatively small thick- 

 ness of the accumulations usually formed by these vents, 

 heir extremely local character, the numerous distinct 

 horizons on which they appear, and the intimate way in 

 which they mingle and alternate with the ordinary Car- 

 boniferous strata, are features which at once arrest the 

 attention of the geologist, presenting, as they do, so 

 striking a contrast to those of the plateaux. 



In a vast number of ancient volcanic vents, no trace 

 can be discovered of their connection with any fissure 

 in the earth's crust (vol. i. p. 53). Such fissures may, 

 indeed, exist underneath, and may have served as 

 passages for the ascent of lava to within a greater or 

 less distance from the surface. But it is established in 

 these volumes that volcanic energy has the power of 

 blowing out an opening for itself through the upper part 

 of the crust without the existence of any visible fissure 

 there. What may be the limits of depth at which this 

 mode of communication with the outer air is possible 

 we do not yet know. They must obviously vary greatly 

 according to the structure of the terrestrial crust on the 

 one hand, and the amount and persistence of volcanic 

 energy on the other. But where the thickness of rock 

 above the end of the fissure is not too great, the ex- 

 pansive energy of the vapours absorbed in the magma 

 may overcome the resistance of that cover, and blow 

 out an orifice by which the volcanic materials can reach 

 the surface. In an ordinary volcanic orifice the ground- 

 plan of the neck is usually irregularly circular or 

 elliptical. 



The discharge of explosive vapours was sometimes 

 the first and only effort of volcanic energy. Generally, 

 however, fragmentary volcanic materials were ejected, 

 and cones of tuff were formed ; or, if the eruption was 

 more vigorous, lava was poured out. Towards the close 

 of a volcanic period, the vents were gradually choked 

 up with the fragmentary materials that were ejected 

 from, and fell back into them. When the vents were 

 plugged up by the consolidation of fragmentary matter, 

 or the uprise of lava in them, the final efforts of the 

 volcanoes led to the intrusion of sills and dykes^ not 

 only into the rocks beneath the volcanic sheets, but also, 

 NO. 1446, AOL.. 56] 



in many instances, into at least the older parts of these 

 sheets themselves. The size and extent of the sills may 

 thus be a record of the intensity of this latest phase of 

 the volcanic eruptions. 



The chief products of the Carboniferous volcanoes are 

 basic rocks, dolerites, and basalts, with andesites ; they 

 are somewhat more acid in the necks, where are found 

 diabases, trachytes, and phonolites. The Puy lavas are 

 generally more basic than the lavas of the plateaux. 



The Permian volcanoes were the last of the long 

 Palaeozoic series, and, so far as we yet know, the whole 

 of the Mesozoic periods, within the area of Britain, were 

 absolutely unbroken by a single volcanic eruption. It 

 was an era of geological calm, during which the Triassic, 

 Jurassic and Cretaceous formations were slowly accumu- 

 lated over the larger part of Europe. The stratigraphical 

 quietude was not, indeed, unbroken. The widespread 

 subsidence of the sea-bottom was interrupted here and 

 there by important upheavals, and considerable geo- 

 graphical changes were in process of time accomplished. 

 But, save in one or two widely separated areas of 

 Europe, there were no active volcanoes over the whole 

 continent. 



After the enormous interval represented by the whole 

 of the Mesozoic and the earlier part of the Tertiary 

 formations, a time of disturbance arose once more, and 

 a surface of 40,000 square miles was covered in the 

 north-west of Britain by great basalt-floods. The whole 

 of these latest volcanic manifestations were comprised 

 within the earlier (Oligocene and, perhaps, early Miocene) 

 part of older Tertiary time. 



The first indications of Tertiary volcanic energy in the 

 north-west of Europe were displayed in the formation of 

 numerous parallel fissures extending in the British Isles 

 in a general north-westerly direction. Between the walls 

 of these opened fissures a basic magma rose and solidified, 

 thus constituting the innumerable dykes of the region. 

 Sometimes the magma reached the surface of the ground, 

 and streaming forth there formed the successive sheets 

 of basalt in the great plateaux. In some instances, as 

 in modern Icelandic eruptions, the lava may have issued 

 immediately from the fissures ; in others its rise has been 

 accompanied with the formation of small cones along 

 the line of a chasm. During a tolerably protracted 

 period, basic and intermediate lavas (basalts, dolerites, 

 andesites, and trachytes) continued to be poured out, 

 together with possibly an occasional outflow of rhyolite. 

 These eruptions took place from many points, and not 

 from great central volcanoes like Vesuvius. The result 

 of their operations was to bury under more than 3000 

 feet of volcanic materials the broad valley between the 

 mainland of Scotland and the chain of the Outer 

 Hebrides. This long series of eruptions is shown to 

 have been subaerial by the terrestrial relics— plants, 

 insects, river-beds and lake-bottoms preserved under 

 and between the lavas. Gradually, as the pile of volcanic 

 material grew in thickness, the magma was less frequently 

 ejected to the surface, but it insinuated itself underneath 

 to form there sills or intrusive sheets. 



The second stage in the Tertiary volcanic history is 

 revealed by the great bodies of amorphous and banded 

 gabbro which form so prominent a feature in the Inner 

 Hebrides. These eruptive masses have disrupted the 



