48 Murchisoris Silurian System. 



active volcanoes, most of the mineral substances composing 

 these lava currents were found to be similar to those of Vesu- 

 vius and Etna. When M. Guettard, one of the naturalists, 

 first announced these discoveries, so unwilling were men of 

 science to believe in phenomena of which neither history 

 nor tradition had preserved a record, that scepticism long pre- 

 vailed. More recent discoveries in Iceland, South America, 

 and Asia Minor have brought to notice all the evidence 

 that can be required to convince us not only of the similarity 

 of modern and ancient volcanic eruptions, says Mr. Murchi- 

 son ; but also of the great extent to which such phenomena 

 have prevailed. " But Auvergne is not merely replete with 

 analogies of modern volcanic regions, it was further found 

 to contain many rocks which though from their characters 

 must have been formed from igneous agency, are yet 

 in many lithological features dissimilar from modern lavas, 

 whilst they resemble many of the so-called trap rocks" ; thus 

 a succession of periods of eruptions and of long intervals of 

 repose have impressed on the various currents of lava and 

 deposits from lakes and rivers, which in a succession of beds 

 occur in central France, such character as to enable the in- 

 quirer to carry backward his researches from the connecting 

 links of existing phenomena, into volcanic operations of high 

 antiquity. 



Mr. Murchison adduces the instance of Graham's island on 

 the coast of Sicily, to prove the vast influence of volcanic agen- 

 cy and the manner in which the results are modified by the 

 sea under our own observation Soundings had proved the sea 

 to be 600 feet deep where the island rose to an elevation 

 of 200 feet above the sea, measuring three miles in circum- 

 ference; yet in three months from its first emergence it 

 again disappeared, and a year after, when the spot was survey- 

 ed, a dangerous reef, eleven feet under water, was all that 

 remained. Thus Mr. Murchison concludes that volcanic 



