PART II. CHAPTER XXIII. 273 



Tests of Relative Age of Volcanic Rocks. 



position in which they were originally deposited in a hollow, 

 existing in the denuded surface formed by the carboniferous 

 strata and intrusive dike. Now here the coal-measures were 

 not only deposited, but had been fissured before the fluid trap 

 was introduced to form the dike. It also appears by the trun- 

 cated edges of the Coal strata, and the abrupt termination of the 

 dike on which the Magnesian limestone rests, that denudation 

 had taken place at a period intervening between the injection of 

 the volcanic matter and the deposition of the Magnesian lime- 

 stone. Even in this case, however, although the date of the vol- 

 canic eruption is brought within Harrow limits, it cannot be 

 defined with precision ; it may have happened either at the close 

 of the carboniferous period, or early in that of the lower New 

 Red sandstone, or between these two periods, when the state of 

 the animate creation, and the physical geography of Europe 

 were gradually changing from the type of the carboniferous era 

 to that of the lower New Red formation. 



In regard to all stratified volcanic tuffs, the test of age by 

 superposition is strictly applicable to them, according to the 

 already explained rules in the case of other sedimentary deposits 

 (See p. 159.) 



Test of age by organic remains. We have seen how, in the 

 vicinity of active volcanos, scoriae, pumice, fine sand, and frag- 

 ments of rock are thrown up into the air, and then showered 

 down upon the land, or into neighbouring lakes or seas. In the 

 tuffs so formed, shells, corals, or any other durable organic 

 bodies which may happen to be strewed over the bottom of a 

 lake or sea, will be imbedded in tuff, and thus continue as per- 

 manent memorials of the geological period when the volcanic 

 eruption occurred. Tufaceous strata thus formed in the neigh- 

 bourhood of Vesuvius, Etna, Stromboli, and other volcanos now 

 active in islands or near the sea, may give information of the 

 relative age of these tuffs at some remote..future period when the 

 fires of these mountains are extinguished. By such evidence we 

 can distinctly establish the coincidence in age of volcanic rocks, 

 and the different primary, secondary, and tertiary fossiliferous 

 strata already considered. 



The tuffs now alluded to are not exclusively marine, but 

 include, in some places, freshwater shells, in others, the bones 

 of terrestrial quadrupeds. The diversity of organic remains in 

 formations of this nature is perfectly intelligible, if we reflect 

 on the wide dispersion of ejected matter during late eruptions, 

 such as that of the volcano of Coseguina, in the province of 

 Nicaragua, January 19, 1835. Hot cinders and fine scoriae 

 were then cast up to a vast height, and covered the ground as 



