120 
NKWER PLIOCENE PERIOD. 
[Ch.X. 
pied in contemplating the gigantic whole, must appear the 
work of extraordinary causes, but when the separate portions 
of which it is made up are carefully studied, they are seen to 
have been formed successively, and the dimensions of each part, 
considered singly, are soon recognized to be comparatively in- 
significant, and it appears no longer extravagant to liken them 
to the recorded effects of ordinary causes. 
Difference in the composition of Somma and Vesuvius. 
As no traditional accounts have been handed down to us of 
the eruptions of the ancient Vesuvius, from the times of the 
earliest Greek colonists, the volcano must have been dormant 
for many centuries, perhaps for thousands of years, previous to 
the great eruption in the reign of Titus. But we shall after- 
wards show that there are sufficient grounds for presuming this 
mountain, and the other igneous products of Campania, to have 
been produced during the Newer Pliocene period. 
We stated in the first volume *, that the ancient and. modern 
cones of Vesuvius were each a counterpart of the other in 
structure; we may now remark that the principal point of 
difference consists in the greater abundance in the older cone 
of fragments of stratified rocks ejected during eruptions. We 
may easily conceive that the first explosions would act with the 
greatest violence, rending and shattering whatever solid masses 
obstructed the escape of lava and the accompanying gases, so 
that great heaps of ejected pieces of sedimentary rock would 
naturally occur in the tufaceous breccias formed by the earliest 
eruptions. But when a passage had once been opened and an 
habitual vent established, the materials thrown out would con- 
sist of liquid Java, which would take the form- of sand and 
scoriae, or of angular fragments of such solid lavas as may have 
choked up the vent. 
Among the angular fragments of solid rock which abound 
in the tufaceous breccias of Somma, none are more common 
than a saccharoid dolomite, supposed to have been derived 
* Chap. xx. 
