PART 11. CHAPTER XXIII. 



275 



Different Ages of Volcanic Rocks. 



Dor, in Auvergne ; but the great mass of trachyte occupies in 

 general an inferior position, and is cut through and overflowed 

 by basalt. It can by no means be inferred that trachyte pre- 

 dominated greatly at one period of the earth's history, and basalt 

 at another, for we know that trachytic lavas have been formed 

 at many successive periods, and are still emitted from many 

 active craters ; but it seems that in each region, where a long 

 series of eruptions have occurred, the more felspathic lavas have 

 been first emitted, and the escape of the more augitic kinds has 

 followed. The hypothesis suggested by Mr. Scrope may, per- 

 haps, afford a solution of this problem. The minerals, he ob- 

 serves, which abound in basalt are of greater specific gravity 

 than those composing the felspathic lavas ; thus, for example, 

 hornblende, augite, and olivine, are each more than three times 

 the weight of water ; whereas common felspar, albite, and La- 

 brador felspar, have each scarcely more than 2^ times the specific 

 gravity of water ; and the dilference 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 traps. 

 If, therefore, a large quantity of rock be melted up in the bowels 

 of the earth by volcanic heat, the denser ingredients of the boil- 

 ing fluid will sink to the bottom, and the lighter remaining above 

 will be first propelled upwards to the surface by the expansive 

 power of gases. Those materials, therefore, which occupied 

 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 superposition alone would be insufficient. It is also 

 not uncommon to find conglomerates almost exclusively com- 

 posed of rolled pebbles of trap, associated with stratified rocks in 

 the neighbourhood of masses of intrusive trap. If the pebbles 

 agree generally in mineral character with the latter, we are then 

 enabled to determine the age of the intrusive rock by knowing 

 that of the fossiliferous strata associated with the conglomerate. 

 The origin of such conglomerates is explained by observing the 

 shingle beaches composed of trap pebbles in modern volcanic 

 islands, or at the base of Etna. 



Recent and newer Pliocene period. — I shall now select ex- 

 amples of contemporaneous volcanic rocks of successive geolo- 

 gical periods, that the reader may be convinced that the igneous 

 causes have been in activity in all past ages of the world, and 



