276 



LYELL'S ELEMENTS OF GEOLOGY. 



Different Ages of Volcanic Rocks. 



that they have been ever shifting the places where they have 

 broken out at the earth's surface. One portion of the lavas, tuffs, 

 and trap-dikes of Etna, Vesuvius, and the island of Ischia, have 

 been produced within the historical era ; another and a far more 

 considerable part have originated at times immediately antece- 

 dent, when the waters of the Mediterranean were already inha- 

 bited by the existing species of testacea. The submarine foun- 

 dations of Etna and Ischia have been upheaved to the great 

 height of between 500 and 1500 feet above the level of the sea.; 

 and the same observations may be made respecting the base of 

 many active volcanos which were first subaqueous vents, or, like 

 Stromboli, half submerged, and then became subaerial, when the 

 ancient bed of the sea was laid dry by elevation. 



Older Pliocene period. — In Tuscany and the Campagna di 

 Roma, submarine volcanic tuffs are interstratified with the Older 

 Pliocene strata of the Subapennine hills in such a manner as to 

 leave no doubt that they were the products of eruptions which 

 occurred when the shelly marls and sands of the Subapennine 

 hills were in the course of deposition. 



Miocene period. — The most ancient volcanic rocks, consisting 

 chiefly of trachyte, of the Upper and Lower Eifel, are interca- 

 lated between Miocene strata in such a manner, as to prove 

 them to have been coeval in origin. The eruptions, however, 

 of the same district were continued down to the Newer Pliocene 

 era, or were at least renewed at that later period, so that show- 

 ers of ashes from the Rhenish volcanos are interstratified with 

 the loess, in which, we have already stated, shells of land and 

 freshwater species occur identical with those now living in 

 Europe.* 



Eocene period. — The extinct volcanos of Auvergne and Can- 

 tal, in central France, commenced their eruptions in the Eocene 

 period, but were most active during the Miocene era. In the 

 lacustrine deposits, near those ancient volcanos, the lowest strata 

 were evidently formed before any eruptions had occurred. They 

 consist of sandstone and conglomerate, containing rounded peb- 

 bles of quartz, mica-schist, granite, and other hypogene rocks, 

 composing the borders of the ancient lakes, but not the slightest 

 intermixture of volcanic products can be detected. To these 

 conglomerates succeeded argillaceous and calcareous marls, 

 containing Eocene shells, during the deposition of which some 

 feeble signs of volcanic action began to show themselves. 

 Above these, freshwater marls and limestones are seen frequently 

 to alternate with volcanic tuff, and in them some fossils of the 



See above, p. 172., and Principles of Geology, book iv. 



