182 DR GEIKTE ON THE HISTORY OF VOLCANIC ACTION 



includes the north of England and of Ireland, the southern half and the west coast of 

 Scotland — a total area of more than 40,000 square miles. Over that extensive region 

 volcanic phenomena were displayed during an enormously protracted interval of 

 geological time. The earliest beginnings of disturbance may possibly go back into the 

 Eocene period, and the final manifestations may not have ceased until the Miocene, or 

 even perhaps later. So protracted was the duration of the eruptions, that there was 

 room for enormous topographical changes from denudation, and also for considerable 

 variation in the fauna and flora, alike of land and sea. 



2. Owing to some cause which has not yet in this relation been investigated, but 

 which is probably referable to secular terrestrial contraction, the volcanic region under- 

 went elevation, while, at the same time, a vast subterranean lake or sea of molten rock 

 appeared underneath it. Enormous horizontal tension thus arose, and at last the 

 stretched terrestrial crust gave way. A system of approximately parallel fissures opened 

 in it, having a general direction towards N.W. The rapid and simultaneous production of 

 such a gigantic series of rents must have given rise to earthquakes of enormous magnitude 

 and destructive force. The great majority of the fractures, doubtless, did not reach to 

 the surface of the ground, though probably not a few did so. Such was the potency of 

 this development of terrestrial energy, that the fissures ran through the most varied 

 kinds of rocks and the most complicated geological structures, crossing even earlier lines 

 of powerful dislocation, and yet retaining their direction and parallelism for sometimes 

 50 or 100 miles. 



3. No sooner were the fissures formed than the molten lava underneath was forced 

 upward into them for many hundreds or even thousands of feet above the surface of the 

 subterranean lava-sea. Solidifying between the fissure walls, it formed the crowd of basic 

 dykes that stands out as the most widespread and distinctive feature of the volcanic region. 



4. Where the fissures reached the surface or near to it, the molten rock would seek 

 relief by egress in streams of lava. This probably occurred in many places from which 

 subsequent denudation has removed all vestige of superficial volcanic manifestations. 

 But, in the great range of basalt-plateaux, from Antrim through the chain of the Inner 

 Hebrides, there are still left abundant remains of the surface outflows. After the con- 

 vulsions ceased which produced the dykes, the communication that had been established 

 between the reservoir of molten rock underneath and the upper air would be maintained, 

 and repeated eruptions might take place either from the original vents or from others 

 afterwards opened by the volcanic energy. 



5. For a prolonged geological period, various basic lavas (basalts, dolerites, &c.) 

 continued to flow out from innumerable vents until they had filled up the hollows of the 

 great valley, which then stretched from the south of Antrim northwards between the 

 west coast of Scotland and the chain of the Outer Hebrides. In some places, the 

 accumulated pile of such ejections even now exceeds three thousand feet, and yet we 

 cannot tell how much material has been bared away from its top by denudation. The 

 surface over which the lava flowed seems to have been mainly terrestrial. Here and there, 



