234 -- -—DYNAMICAL GEOLOGY. ~ [Boo IIL 

under a new tumultuous deposit by which the surface is greatly — 
changed, not only as regards its temporary aspect, but in its more — 
permanent features, such as the position and form of its water- — 
courses. 
Mud-Volcanoes.—These are of two kinds: 1st, where the chief 
source of movement is the escape of gaseous discharges; 2nd, where 
the active agent is steam. 
(1) Although not volcanic in the proper sense of the term, certain 
remarkable orifices of eruption may be noticed here, to which the 
names of mud-volcanoes, salses, air-voleanoes, and macalubas have been 
applied. ‘These are conical hills formed by the accumulation of fine 
and usually saline mud, which, with various gases, is continuously 
or intermittently given out from the orifice or crater in the centre, 
They occur in groups, each hillock being sometimes less than a yard 
in height, but ranging up to elevations of 100 feet or more. Like true 
volcanoes, they have their periods of repose, when either no discharge 
takes place at all, or mud oozes out tranquilly from the crater, and 
their epochs of activity, when large volumes of gas, and sometimes 
columns of flame, rush out with considerable violence and explosion, 
and throw up mud and stones to a height of several hundred feet. 
The gases play much the same part, therefore, in these phenomena 
that steam does in those of true voleanoes. They consist of carbon 
dioxide, carburetted hydrogen, sulphuretted hydrogen, and nitrogen. 
The mud is usually cold. In the water occur various saline in- 
egredients, among which common salt generally appears; hence the 
name, Salses. Naphtha is likewise frequently present. Large pieces 
of stone, differing from those in the neighbourhood, have been 
observed among the ejections, indicative doubtless of a somewhat 
deeper source than in ordinary cases. Heavy rains may wash down 
the minor mud-cones and spread out the material over the ground, 
but gas-bubbles again appear through the sheet of mud, and by 
degrees a new series of mounds is once more thrown up. 
There can be little doubt that this type of mud-voleano is to be 
traced to chemical changes in progress underneath. Dr. Daubeny 
explained them in Sicily by the slow combustion of beds of sulphur. 
The frequent occurrence of naphtha and of imflammable gas points, 
in other cases, to the disengagement of hydrocarbons from subter- 
ranean strata. 
(2) The second class of mud-voleano presents itself in true volcanic 
regions, and is due to the escape of hot water and steam through 
beds of tuff or some other friable kind of rock. The mud is kept in 
ebullition by the rise of steam through it. As it becomes more 
pasty and the steam meets with greater resistance, large bubbles are 
formed, which burst, and the more liquid mud from below oozes out 
from the vent. In this way small cones are built up, many of — 
which have perfect craters atop. In the Geyser tracts of the Yellow- 
stone region there are several instructive examples of such active 
and extinct mud-vents. Some of the extinct cones there are not ~ 

