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240 DYNAMICAL GEOLOGY. 
their liquid and solid accompaniments make their way to the surface — 
and gradually build up a volcanic hill or mountain. Occasionally 
eruptions have proceeded no further than the first stage of gaseous 
explosion. A cauldron-like cavity has been torn open in the ground, © 
and ejected fragments of the solid rocks through which the explosion — 
has emerged have fallen back into and round the vent. Subse- 
quently, after possible subsidence of the fragmentary materials in the 
vent, and even of the sides of the orifice, water supplied by rain and 
filtering from the neighbouring ground has partially, or wholly, filled 
up the cavity. In this way a lake has arisen either with or without 
a superficial outlet. Under favourable circumstances, vegetation 
creeping over bare earth and stone, may so conceal all evidence of 
the original volcanic action as to make the quiet sheet of water look 
as if it had always been an essential part of the landscape. Explosion 
lakes of this kind occur in districts of extinct volcanoes as in the Hifel 
(maare), central Italy, and Auvergne. A remarkable example is 
supplied by the Lonar Lake in the Indian peninsula, half-way 
between Bombay and Nagpur. It lies in the midst of the volcanic 
plateau of the Deccan traps, which extend around it for hundreds of ~ 
miles in nearly flat beds that slightly dip away from the lake. An 
almost circular depression, rather more than a mile in diameter, and 
from 300 to 400 feet deep, contains at the bottom a shallow lake of | 
bitter saline water, depositing crystals of trona (sesquicarbonate of 
soda). Except to the north and north-east, it is encircled with a raised — 
rim of irregularly piled blocks of basalt, identical with that of the beds 
through which the cavity has been opened. The rim never exceeds 
100 feet, and is often not more than 40 or 50 feet in height, and 
cannot contain a thousandth part of the material which once filled 
the crater. No other evidence of volcanic discharge from this vent 
is to be seen. Some of the contents of the cavity may have been 
ejected in finer particles, which have subsequently been removed by 
denudation ; but it seems more probable that the existence of the 
cavity is mainly due to subsidence after the original explosion. 
In most cases explosions are accompanied by the expulsion of so: 
much solid material that a cone gathers round the point of emission. 
As the cone increases in height by successive additions of ashes or 
lava to its surface, these volcanic sheets are laid down upon pro- 
gressively steeper slopes. ‘The inclination of beds of lava, which 
must have originally issued in a more or less liquid condition, offered 
formerly a difficulty to observers, and suggested the famous theory of 
Hlevation-craters (Hrhebungskratere) of L. von Buch,? E. de Beaumont,’ 
and other geologists. According to this theory the conical shape of 
a volcanic cone arises mainly from an upheaval or swelling of the 
' On explosion-craters and lakes, see Scrope’s Volcanoes. Lecoq, Epoques Géologiques — 
de V Auvergne, tome iy.; compare also Vogelsang, Vulcane der Hifel, and in Neues ~ 
Jahrb. 1870; pp. 199, 326, 460. On Lonar Lake, see Malcolmson, Trans. Geol. Svc.’ 
2nd ser., vy. p. 562, Medlicott and Blandford’s “ Geology of India,” p. 379, 
2 Pogg. Ann, ix., X., XXXVil. p. 169, 
* Bull. Soc. Geol. France, iv. p. 357. Ann, des Mines, ix. and x. 
