522 EELATIVE AGES OF VOLCANIC EOCKS. [On. XXX. 



fragments by denudation, we might still perhaps identify the detached 

 portions by their similarity in mineral composition. Nevertheless, this 

 test will not always avail the geologist ; for, although there is usually a 

 prevailing character in lava emitted during the same eruption, and even 

 in the successive currents flowing from the same volcano, still, in many 

 cases, the different parts even of one lava-stream, or, as before stated, of 

 one continuous mass of trap, vary much in mineral composition and 

 texture. 



In Auvergne, the Eifel, and other countries where trachyte and basalt 

 are both present, the trachytic rocks are for the most part older than the 

 basaltic. These rocks do, indeed, sometimes alternate partially, as in the 

 volcano of Mont Dor, in Auvergne ; and we have seen that in Madeira 

 trachytic rocks may overlie an older basaltic series (p. 517) ; but the great 

 mass of trachyte occupies more generally, perhaps, an inferior position, and 

 is cut through and overflowed by basalt. It can by no means be inferred 

 that trachyte predominated 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, perhaps, afford a solution of this problem. The minerals, 

 he observes, 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 Labrador felspar, have each scarcely 

 more than 21 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 

 occupied the lowest place in the subterranean reservoir will always be 

 emitted last, and take the u])permost place on the exterior of the earth's 

 crust. 



Test by included fragments, — We may sometimes discover the rela- 

 tive 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 a conglomerate 

 almost exclusively composed of rolled pebbles of trap, associated with 

 some fossiliferous stratified formation in the neighborhood of massive 

 trap. If the pebbles agree generally in mineral character with the 

 latter, we are then enabled to determine its relative age by knowing 

 that of the fossiliferous strata associated With, the conglomerate. The 



