VOLCANIC CRATERS AND EXPLOSIONS." 
By TEMPEST ANDERSON, D.Sc. 
(From 1 The Geographical Journal'for February, 1912.) 
The subject of Volcanoes naturally divides itself into two branches—the 
Physiographical, or naked-eye one, and the Petrological, which latter deals 
with the chemical and microscopical structure of the rocks produced. 
This branch has by itself provided the subject of life-studies by such men 
as Sorby, Bonney, Teall, in this country, to say nothing of foreign investi¬ 
gators almost equally distinguished ; but it is extremely difficult and 
complicated, and I do not propose to discuss it to-night. 
The other branch deals with the physiographical phenomena of volcanic 
cones, craters, and lava streams ; with deposits of pumice, tuff, ash beds ; 
with dykes, sills and intrusive sheets, their structure and mode of pro¬ 
duction, and their alteration by weathering and other forms of denudation. 
It may fairly be included in physical geography, and is, therefore, suitable 
for consideration by this Society. Even this is far too large a subject to 
be dealt with in a single lecture, so to-night I shall confine myself chiefly 
to one division of it, viz. Craters and Explosions, and only mention the 
other branches incidentally and by way of illustration. 
If we watch most volcanic eruptions, small enough to be safely 
approached, such for instance as that of Vesuvius in September, 1898 ,f 
we see a discharge from the crater of vast quantities of dust and ashes 
suspended in steam and various volcanic gases, while lower down the cone 
is a discharge of lava, building up in this case a hill, Colle Umberto primo. 
These mark two very distinct factors, and according as one or other 
predominates, the eruption may be classed as of the explosive or efflusive 
type. Thus the eruption of the Soufriere in St. Vincent in 1902 was wholly 
explosive, no lava being discharged, while in those of the Hawaiian and 
* Royal Geographical Society, November 20, 1911. 
f Slide shown. 
