RELATION OF VOLCANOES TO STRATIFED ROCKS. 179 



Extinct Volcanoes. Many examples exist of volcanoes which 

 have so long ceased to be active that they are regarded as extinct, 

 and we must attribute the failure of their eruptive power in most 

 cases either to changed conditions in the relations of land and water, 

 which have deprived the countries where they occur of the requisite 

 water supply for the generation of steam, or to the exhaustion of 

 rock-material beneath the surface, which was capable of producing 

 ashes and lava, or else, as is probably frequently the case, to changes 

 in the direction of the grand contractions beneath the surface, by which 

 the subterranean energy became transferred from a region where it was 

 formerly manifested, to a new locality changes which have taken place 

 in all past periods of geological time, with the result of altering the 

 distribution of sediments, and of life, as well as of volcanoes. 



Relation of Volcanoes to Stratified Kocks. Finally, the mate- 

 rials which volcanoes bring to the surface remain to be examined It 

 is well known that though these present in the rock-substance every 

 variety in texture, yet at different times, and in different regions, 

 volcanoes pour out ashes and lava which differ materially in their 

 mineral and chemical composition. As we have already observed, 

 rocks which are poor in silica, like basalt and the leucite basalt of 

 Vesuvius, form a group which has been termed basic, while the 

 trachytes and rhyolites, which are rich in silica, form another series 

 of volcanic rocks. Why these two groups, which were recognised by 

 most of the early masters in geology, should exist and alternate, is a 

 problem that can only be estimated by recognising that, the alterna- 

 tion has existed for all geological time. But we might fail altogether 

 to elucidate this problem unless we observe that, some of the ancient 

 volcanic cones in the Eifel, as remarked by Professor Judd, are largely 

 made up of fragments of slate, which have been ejected from the 

 vents by explosive forces. And it is well known that the surface 

 of Vesuvius is covered with fragments of limestone, ejected from the 

 throat of the volcano ; and many of these blocks are so little altered 

 that Professor Guiscardi has been able to recognise several hundred 

 species of shells in these masses. We thus discover that, deep beneath 

 many volcanoes, stratified rocks exist; and as we are compelled to 

 believe that the plutonic rocks were metamorphosed out of such strati- 

 fied materials, so we find no anomaly in the basis of a volcano being 

 formed by the liquefication of similar strata, nor indeed is there any 

 other probable explanation available for the diversity of lavas ejected. 

 All the phenomena which are connected with the existence of vol- 

 canoes are hence related to each other in a sequence, which owes its 

 existence primarily to the cooling of the earth's crust. 



