056 



RELATIVE AGE OF VOLCANIC ROCKS. 



[Oh. XXX. 



Fig. 711. 



sive, or if, in pursuing b for some distance, we find at length that it 

 cuts through the stratum a, and then overlies it as at e. 



We may, however, be easily deceived in supposing the volcanic 

 rock to be intrusive, when in reality it is contemporaneous ; for a 

 sheet of lava, as it spreads over the bottom of the sea, cannot rest 

 everywhere upon the same stratum, either because these have been 

 denuded, or because, if newly thrown down, they thin out in certain 

 places, thus allowing the lava to cross their edges. Besides the 

 heavy igneous fluid will often, as it moves along, cut a channel into 



beds of soft mud and sand. Suppose 

 the submarine lava f (fig. Yll) to have 

 come in contact in this manner with 

 the strata, a, b, c, and that after its 

 consolidation the strata, d, e, are thrown 

 down in a nearly horizontal position, 

 yet so as to lie unconformably to f, the 

 appearance of subsequent intrusion will 

 here be complete, although the trap is in fact contemporaneous. We 

 must not, therefore, hastily infer that the rock f is intrusive, unless 

 we find the strata, d, e, or c, to have been altered at their junction, 

 as if by heat. 



The test of age by superposition is strictly applicable to all stratified 

 volcanic tuffs, according to the rules already explained in the case of 

 other sedimentary deposits (see p. 93). 



Test of Age by Organic Remains. — We have seen how, in the vicinity 

 of active volcanoes, scoriae, pumice, fine sand, and fragments of rock 

 are thrown up into the air, and then, showered down upon the land, 

 or into neighboring 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, and thus continue 

 as permanent memorials of the geological period when the volcanic 

 eruption occurred. Tufaceous strata thus formed in the neighborhood 

 of Vesuvius, Etna, Stromboli, and other volcanoes 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 moun- 

 tains are extinguished. By evidence of this kind we can establish a 

 coincidence in age between volcanic rocks and the different primary, 

 secondary, and tertiary fossiliferous strata. 



The tuffs alluded to may not always be marine, but may include, 

 in some places, freshwater shells ; in others, the bones of terrestrial quad- 

 rupeds. 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 

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

 they fell to the depth of more than 10 feet and for a distance of 8 

 leagues from the crater in a southerly direction. Birds, cattle, and 



