~ 

Parr I. Sect. i. §3.] VOLCANIC CONES. - 249 
While, therefore, a volcano, and more particularly one of great 
size, throwing out both lava and fragmentary 
materials, is liable to continual modification of 
its external form as the result of successive 
erlptions, its contour is likewise usually ex- 
posed to extensive alteration by the effects of 
ordinary atmospheric erosion as well as from 
the condensation of the volcanic vapours. 
_ Heavy and sudden floods produced by the 
rapid rainfall consequent upon a copious dis- 
charge of steam, rush down the flanks of a ) | 
volcano with such volume and force as to cut 
deep gullies in the loose or only partially con- 
- solidated tuffs and scorize. Ordinary rain con- 
tinues the erosion until the outer slopes, unless ey 
occasionally renewed by fresh showers of de- = 
tritus, assume a curiously furrowed aspect, like 
a half-opened umbrella, the furrows being | 
separated by ridges that narrow upwards to- 
wards the summit of the cone. ‘The outer 
declivities of Monte Somma afford an excel- 
lent illustration of this form of surface, the 
numerous ravines on that side of the mountain 
presenting instructive sections of the prehis- 
yl 
} 
g 
toric lavas and tuffs of the earlier and more ~ 
Sty 
yee 
_m 
2, edges of 
; 1, Camaldoli (534) ; m, Torre dell’ Annunziata, on Bay of Naples. 
aroni (1588) ; e, Punta 
9; 
Atrio del Cavallo, bottom of old crater of Monte Somma ; 
) of Vesuvius (3640); &, Boeche Nuove (1515) 

dell’ Olivella (994); d, Casa Canc 
); ¢c, Fontana 
Sf 
important period in the history of this volcano. 
Similar trenches have been eroded on the 
southern or Vesuvian side of the original cone, 
but these have in great measure been filled ~~ 
up by the lavas of the younger mountain, 
The ravines in fact form natural channels for 
the lava, as may unfortunately be seen round E 
the Vesuvian observatory. The building was 
placed on one of the ridges between two deep © 
ravines ; but the lava streams of recent years | 
have poured into these ravines on either side 
and are rapidly filling them up, 
_ 6. Submarine Voleanoes.—It is not only 
on the surface of the land that volcanic 
action shows itself. It takes place likewise = 
under the sea, and as the geological records of 
the earth’s past history are chiefly marine for- 
mations, the characteristics of submarine vol- 
eanic action have no small interest for the 
geologist. In a few instances the actual out- 
break of a submarine eruption has been wit- 
nessed. Thus in the early summer of 1783 a’ 
volcanic eruption took place about thirty miles from Cape Reykjanaes 
Village of Somma (413 French feet 
Fic. 56.—Srcrion oF VEsuvius AND MontEe Somma FRom NortH To Sours. 
f old crater of Monte Somma; 
ae 
di Nasone (3430), on crest o 
a, Plain of the Campagna; J, 
modern crater (h 
