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. [Sidenote: ANCIENT BRITISH VOLCANOES.]