OF MONTE SOMMA AND VESUVIUS. 65 
outflow of lava and the next grand paroxysmal eruption. This 
strikes us still more when we remember the time taken to cover a 
lava-stream with a vegetable soil nearly a metre in thickness. That 
it was a surface, and therefore in all probability supporting vege- 
tation, is proved by the advanced state of kaolinization and general 
decomposition of the enclosed lava fragments, brought about by their 
exposure to frost and heat, combined with moisture resulting from 
their exposed position. Where the lavas or scoriz of PHasz IV. 
are absent, these vegetable soils may represent, in time, that phase 
as well as Puase V. 
Past VI. 
Under this heading are grouped a series of eruptions remarkable 
for their paroxysmal violence, and the pumiceous nature of the 
ejyectamenta, mixed with vast quantities of the leucilitic nucleus of 
the mountain, together with its sedimentary foundation-rocks. The 
first eruption of the group is perhaps one of the most violent that 
ever shook the flanks of this classical volcano, and the last must have 
far outrivalled the great eruption that destroyed Herculaneum and 
Pompeii, which succeeded it. 
Puass VI., Period 1. 
General Description.—The ejectamenta of this eruption form a 
stratum which is one of the thickest and most constant we en- 
counter on the slopes of Monte Somma. Its peculiar lithological 
and petrological characters render its recognition so easy, that it 
forms an important datum-line in the determination of the position 
of more dubious deposits occurring above or below it. It rests often 
unconformably on strata of earlier date; but where these have not 
been eroded, it may seem quite conformable with them. 
Generally speaking, the materials forming this deposit are derived 
from three entirely different sources. It will be most convenient to 
describe these separately, and then compare their importance as to 
bulk in constituting the whole of the ejectamenta. 
The first and most worthy of study is the primary, or essential 
eruptive matter. It is a hard, moderately compact pumice, the 
major part of which will sink in water. It is tough, and consider- 
ably resists the blow of a hammer. Its colour is usually brownish, 
ranging from a light buff to almost black. Large cellular cavities 
are occasionally produced in it by the union of numerous pores, but 
more frequently by the presence of fragments of limestone, around 
which they have formed. The pieces of calcareous rock are covered 
with a white crumbling coat, probably the superficial portion that 
was converted into caustic lime by the high temperature of the mass 
surrounding it. The escaping carbonic anhydride would form a 
vesicle, against the side of which would remain attached the bit of 
limestone. Possibly these calcareous fragments may have had a 
catalytic action, just as we see any rough mass, such as bread, 
plunged into champagne bring about a rapid evolution of gas upon 
its surface. Or we may roughly compare the action to that of a 
morsel of yeast in bread, which may often be noticed attached to 
Q.J.G.8. No. 157. F 
