522 
ELEMENTS OF GEOLOGY. 
or if, in pursuing h 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 contempo¬ 
raneous ; for a sheet of lava, as it spreads over the bottom 
of the sea, can not 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. Be¬ 
sides, 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. 598) to have come in con¬ 
tact in this manner with the strata 
a, 6, c, and that after its consolida¬ 
tion the strata e are thrown down in a nearly horizontal 
position, yet so as to lie unconformably to F, the appearance 
of subsequent intrusion Avill 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 
overlying strata, 6, 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 sedimentary deposits (see p. 124). 
Test of Age by Organic Eemains. —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 tufis so formed shells, corals, or any other dura¬ 
ble 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 in islands or near the sea, may give informa¬ 
tion of the relative age of these tuffs at some remote future 
period when the fires of these mountains are extinguished. 
By evidence of this kind we can establish a coincidence in 
age between volcanic rocks and the different primary, sec¬ 
ondary, and tertiary fossiliferous strata. 
The tuffs alluded to may not always be marine, but may 
include, in some places, fresh - water shells; in others, the 
bones of terrestrial quadrupeds. The diversity of organic 
remains in formations of this nature is perfectly intelligible, 
Fig. 598. 
