120 NEWER 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 lava, 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. 



