658 RELATIVE AGE OF VOLCANIC ROCKS. [Oh. XXX. 



water ; whereas common felspar, albite, and Labrador felspar have each 

 scarcely more than 2-| times the specific gravity of water; and the 

 difference is increased in consequence of there being much more iron 

 in a metallic state in basalt and greenstone than in trachyte and other 

 felspathic lavas and trap rocks. If, therefore, a large quantity of rock 

 be melted up in the bowels of the earth by volcanic heat, the denser 

 ingredients of the boiling fluid may sink to the bottom, and the 

 lighter remaining above would in that case be first propelled upwards 

 to the surface by the expansive power of gases. Those materials, 

 therefore, which occupy the lowest place in the subterranean reservoir 

 will always be emitted last, and take the uppermost place on the 

 exterior of the earth's crust. 



Test by Included Fragments. — : We may sometimes discover the 

 relative age of two trap rocks, or of an aqueous deposit and the trap 

 on which it rests, by finding fragments of one included in the other in 

 cases such as those before alluded to, where the evidence of super- 

 position alone would be insufficient. It is also not uncommon to find 

 a conglomerate almost exclusively composed of rolled pebbles of trap, 

 associated with some fossiliferous stratified formation in the neighbor- 

 hood of massive trap. If the pebbles agree generally in mineral char- 

 acter with the latter, we are then enabled to determine its relative age 

 by knowing that of the fossiliferous strata associated with the con- 

 glomerate. The origin of such conglomerates is explained by observ- 

 ing the shingle beaches composed of trap pebbles in modern volcanic 

 islands, or at the base of Etna. 



Newer Tertiary Pliocene Periods. — I shall now select examples of 

 contemporaneous volcanic rocks of successive geological periods, to 

 show that igneous causes have been in activity in all past ages of the 

 world, and 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 has been produced within the historical era ; 

 another and a far more considerable part originated at times imme- 

 diately antecedent, when the waters of the Mediterranean were already 

 inhabited by the existing testacea, but when certain species of 

 elephant, rhinoceros, and other quadrupeds now extinct, inhabited 

 Europe. A third and more ancient portion again of these volcanoes 

 originated at the close of the Newer Pliocene period, when less than 

 ten, sometimes only one, in a hundred of the shells differed from those 

 now living (see p. 190). 



: . It has already been stated that in the case of Etna, Post-pliocene 

 formations occur in the neighborhood of Catania, while the oldest 

 lavas' .of the great volcano are Pliocene. These are seen associated 

 with sedimentary deposits at Trezza and other places on the southern 

 and eastern flanks of the great cone (see above, p. 190). 



The Cyclopian Islands, called by the Sicilians Del Farraglioni, in 

 the sea-cliffs of which these beds of clay, tuff, and associated lava are 



