SHIFTING OF VOLCANIC ACTION IN DIFFERENT AREAS.
Having then considered the arguments which have been adduced in support of the view that the volcanic phenomena of former geological periods differ from those which are still occurring upon the globe, we may proceed to state the general conclusions which have been drawn from the study of the volcanic rocks of the different geological periods.
From a survey of the volcanic rocks of different ages, we are led to the interesting and important conclusion that the scene of volcanic action has been continually shifting to fresh areas at different periods of the earth's history. We find repeated proofs that the volcanic energy has made its appearance at a certain part of the earth's crust, has gradually increased in intensity to a maximum, and then as slowly declined. But as these manifestations have died away at one part of the earth's surface, they have gradually made their appearance at another. In every district which has been examined, we find abundant proofs that volcanic energy has been developed at certain periods, has disappeared during longer or shorter periods, and then reappeared in the same area. And on the other hand, we find that there is no past geological period in which we have not abundant evidence that volcanic outbursts took place at some portion of the earth's surface.
To take the case of our own islands for example. We know that during the pre-Cambrian periods volcanic outbursts occurred, traces of which are found both in North and South Wales, in the Wrekin Chain in Shropshire, in Charnwood Forest, and in parts of Scotland and Ireland.
In Cambro-Silurian times we have abundant proofs, both in North Wales and the Lake district, that volcanic action on the very grandest scale was taking place during the Arenig and the older portion of the Llandeilo periods, and again during the deposition of the Bala or Caradoc beds. The lavas, tuffs, and volcanic agglomerates ejected during these two periods have built up masses of rock many thousands of feet in thickness. Snowdon and Cader Idris among the Welsh mountains, and some of the higher summits of the Lake district, have been carved by denudation from the vast piles of volcanic materials ejected during these periods.
In Devonian or Old-Red-Sandstone times, volcanic activity was renewed with fresh violence upon that part of the earth's surface now occupied by the British Islands. Along the line which now forms the Grampians there rose a series of volcanoes of the very grandest dimensions. Ben Nevis, and many others among the higher Scotch mountains, have been carved by denudation from the hard masses of granite, quartz-felsite, and other plutonic rocks which formed the central cores of these ancient volcanic piles. The remains of the great lava-sheets, and of the masses of volcanic agglomerate ejected from these grand Devonian volcanoes, make up hill-ranges of no mean altitude, like the Sidlaws, the Ochils, and the Pentlands.
ANCIENT BRITISH VOLCANOES.
The volcanic action of the Devonian period was prolonged into Carboniferous times, but was then evidently diminishing gradually in violence. Instead of great central volcanoes, such as existed in the earlier period, we find innumerable small vents which threw out tuffs, agglomerates and lavas, and were scattered over the districts lying around the bases of the now extinct Devonian volcanoes. In the central valley of Scotland and in many parts of England, we find abundant proofs of the existence of these small and scattered volcanic vents during Carboniferous times. The well-known hill of Arthur's Seat, which overlooks the city of Edinburgh, and many castle-crowned crags of the Forth and Clyde valleys, are the worn and denuded relics of these small volcanoes. There are some indications which point to the conclusion that the volcanic action of the Newer Palæozoic epoch had not entirely died out in Permian times, but the evidence upon this point is not altogether clear and satisfactory.
During nearly the whole of the Secondary or Mesozoic periods the volcanic forces remained dormant in the area of the British Isles. Some small volcanic outbursts, however, appear to have occurred in Triassic times in Devonshire. But in other areas, such as the Tyrol, South-eastern Europe and Western America, the Triassic, Jurassic, and Cretaceous periods were marked by grand manifestations of volcanic activity.
The volcanic forces which had during the long Mesozoic periods deserted our part of the earth's surface, appear to have returned to it in full rigour in the Tertiary epoch. In the Newer-Palæozoic periods the direction of the great volcanic band which traversed our islands appears to have been from north-east to south-west; but in Tertiary times a new set of fissures were opened running from north to south. There is evidence that during the Eocene or Nummulitic period, the first indications of the subterranean forces having gathered strength below the district were afforded by the issue of calcareous and siliceous springs, and soon fissures were opened which emitted scoriæ, tuffs, and lavas. The intensity of the volcanic action gradually increased till it attained its maximum in the Miocene period, when a great chain of volcanic mountains stretched north and south along the line of the Inner Hebrides, the north-east of Ireland, and the sea which separates Great Britain from Ireland. The basal-wrecks of a number of these volcanoes can be traced in the islands of Skye, Mull, Rum, and parts of the adjoining mainland. We have already seen that along this great band of volcanic action, which traverses the Atlantic Ocean from north to south, a number of active vents still exist, though their energy is now far less intense than was the case in former times. The only vestiges of the action of these now declining volcanic forces, at present found in our islands, are the hot springs of Bath and a few other warm and mineral springs; but in connection with this subject it must be remembered that our country occasionally participates in great earthquake-vibrations, like that which destroyed Lisbon in the year 1759.