Ch. XXX.] THE POST-PLIOCEXE PERIOD. 661 



Post-Pliocene Formations near Naples. — I have traced in the 

 " Principles of Geology " the history of the changes which the vol- 

 canic region of Campania is known to have undergone during the last 

 2000 years. The aggregate effect of igneous operations during that 

 period is far from insignificant, comprising as it does the formation of 

 the modern cone of Vesuvius since the year 79, and the production of 

 several minor cones in Ischia, together with that of Monte jNTuovo in 

 the year 1538. Lava-currents have also flowed upon the land and 

 along the hottom of the sea — volcanic sand, pumice, and scoriae have 

 been showered down so abundantly that whole cities were buried — 

 tracts of the sea have been filled up or converted into shoals- — and 

 tufaceous sediment has been transported by rivers and land-floods to 

 the sea. There are also proofs, during the same recent period, of a 

 permanent alteration of the relative levels of the land and sea in sev- 

 eral places, and of the same tract having, near Puzzuoli, been alter- 

 nately upheaved and depressed to the amount of more than 20 feet. 

 In connection with these convulsions, there are found, on the shores 

 of the Bay of Baias, recent tufaceous strata, filled with articles fabri- 

 cated by the hands of man, and mingled with marine shells. 



It was also stated in this work (p. 189), that when we examine this 

 same region, it is found to consist largely of tufaceous strata, of a date 

 anterior to human history or tradition, which are of such thickness 

 as to constitute hills from 500 to more than 2000 feet in height. 

 Some of these strata contain marine shells which are exclusively of 

 living species, others contain a slight mixture, one or two per cent., 

 of extinct species. Of the latter class is the ancient cone of Vesuvius, 

 called Somma, which is of far greater volume than the modern cone, 

 and is intersected by a far greater number of dikes. In contrasting 

 this ancient part of the mountain with that of modern date, one prin- 

 cipal point of difference is observed : namely, the greater frequency in 

 the older cone of fragments of altered sedimentary 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 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 consist 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 fragments which abound in the tufaceous breccias of 

 Somma, none are more common than a saccharoid dolomite, supposed 

 to have been derived from an ordinary limestone altered by heat and 

 volcanic vapors. 



Carbonate of lime enters into the composition of so many of the 

 simple minerals found in Somma, that M. Mitscherlich, with much 

 probability, ascribes their great variety to the action of the volcanic 

 beat on subjacent masses of limestone. 



