Page:A Treatise on Geology, volume 2.djvu/226

 212 energy of the subterranean fires, in ejecting matter to the surface, becomes most astonishing. The fearful eruption alluded to did not entirely cease till the end of two years: in its course the lava filled valleys 600 feet in depth; dried and took the place of lakes; accumulated in rocky gorges; spread in wide plains till they became broad burning lakes, sometimes from 12 to 15 miles wide, and 100 feet deep. The lava may be said to have taken two principal and nearly opposite directions; flowing in one 50, and in the other 40 miles, with a breadth in the former case of 15 miles, in the latter of 7. The ordinary depth of the accumulated mass was about 100 feet, but in narrow defiles it sometimes amounted to 600 feet. Mr. Lyell, from whose admirable summary of this destructive eruption the above abstract is taken, makes an ingenious comparison of this prodigious mass of modern pyrogenous rock with older effects of the interior heat of the globe, and illustrates its effect on the geology of England, if spread like the basaltic plateau of Antrim. Spread upon the stratified rocks of England, before their elevation from the sea bed, the lava would have occupied a vast continuous surface; and, after the rising of the rocks and their waste by watery action, the original extent might be traced. The Skápta branch of the lava might rest on the high oolitic escarpment which commands the vale of Gloucester, 100 feet in thickness, and from 10 to 15 miles broad, exceeding any which can be found in Central France. Great tabular masses might occur at intervals, capping the summit of the Cotswold hills, between Gloucester and Oxford, by Northleach, Burford, and other towns. The same rocks might recur on the summit of Cumnor and Shotover hills, and all the other oolitic eminences of that district. Plateaus 6 or 7 miles wide might have crowned the chalk of Berkshire, and masses 500 or 600 feet thick might have raised the hills of Highgate and Hampstead to rival or surpass Salisbury Craigs and Arthur's Seat. (Principles of Geol. book ii. ch. xii.)