524 
ELEMENTS OF GEOLOGY. 
basalt. It can by no means be inferred that trachyte pre¬ 
dominated 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 lavas containing 
feldspar more rich in silica have been first emitted, and the 
escape of the more augitic kinds has followed. The hypoth¬ 
esis suggested by Mr. Scrope may, perhaps, afibrd a solution 
of this problem. The minerals, he observes, which abound 
in basalt are of greater specific gravity than those composing 
the feldspathic lavas ; thus, for example, hornblende, augite, 
and olivine are each more than three times the weight of 
water; whereas common feldspar and albite have each scarce¬ 
ly more than 2 J times the specific gravity of water; and the 
difierence is increased in consequence of there being much 
more iron in a metallic state in basalt and greenstone than 
in trachyte and other allied feldspathic lavas. 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 
fiuid may sink to the bottom, and the lighter remaining 
above would in that case be first propelled upward to the 
surface by the expansive power of gases. Those materials, 
therefore, which occupy 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 discov¬ 
er the relative age of two trap-rocks, or of an aqueous de¬ 
posit 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 insuf¬ 
ficient. It is also not uncommon to find a conglomerate al¬ 
most exclusively composed of rolled pebbles of trap, asso¬ 
ciated with some fossiliferous stratified formation in the 
neighborhood of massive trap. If the pebbles agree gener¬ 
ally in mineral character with the latter, we are then ena¬ 
bled to determine its relative age 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 volca¬ 
noes, as at the base of Etna, 
Recent and Post-pliocene Volcanic Rocks.— I shall now se¬ 
lect examples of contemporaneous volcanic rocks of succes¬ 
sive geological periods, to show that igneous causes have been 
in activity in all past ages of the world. They have been 
perpetually shifting the places where they have broken out 
