204 Records of the Geological Survey of India. [vol. xi. 
the siirface; and 3rd, the presence of a power competent to force it up to the 
mouth of the Tent and leave it there to reconsolidate. In the case of ‘true’ 
volcanoes the mohility is in the main gained hy the reduction of rock to a state 
of fusion, the heat involved in the process being due, as has been shown by 
Mr. R. Mallet, to local crushing of the earth’.s crust, caused hy the secular refri¬ 
geration of the globe, the interior cooling, and consequently contracting, faster 
than the exterior, which is thus- subjected to tangential thrusts. The force hy 
which the lava is ejected is the pressure of steam raised to a high temperature 
hy heat derived from the same source as that which fuses the rock.^ 
In the case of salses the mobility is produced hy reduction of rock to a 
state of mud, either by partly chemical, or by purely mechanical means.* * The 
ejecting force is sometimes the pressure of steam; in other cases, including the 
Arrakan salses, it is, mainly at least, that of gaseous hydrocarbons. 
Amongst the most prominent examples of steam mud volcanoes are those in 
New Zealand, described by Dr. Hochstetter, where are craters filled with boiling 
mud from which steam escapes explosively at intervals, accompanied by sulphuric 
acid and other gases, to whose action on the lavas underground Dr. Hochstetter 
attributes the origin of the mud.^ Of a similar character are the boiling mud 
volcanoes of Iceland, which have been described by Professor Paijjull, Captain 
C. S. Forbes, and others. Such also are the mud volcanoes of the Colorado 
desert. From some of these “the steam rashes in a continuous stream, with a 
roaring or whizzing sound, as the orifices vary in diameter or the jets differ in 
velocity. In others the action is intermittent, and each recurring rush of steam 
is accompanied by a discharge of a shower of hot mud, masses of which are 
thrown sometimes to the height of a hundred feet.”* 
In all the above cases the mud cones are in the closest relationship with 
existing, or but recently extinct, phenomena of volcanic fusion; the mud is pro¬ 
duced by chemical action of acid gases on volcanic rocks, and the steam is 
generated by the heat which originally fused these, or by heat which has been 
produced in the same way. The heat required, however, is of a much lower 
degree of intensity, and hence such steam mud volcanoes may long outHve the 
immeasurably grander phenomena of the lava-emitting period. It is of course 
necessary that the springs should be boiling, steam at considerable pressure, but 
whose force is dissipated at the vent, being the power involved. A spring of 
water merely, whether hot or cold, bearing a certain proportion of mud to the 
surface, would be equally capable of carrying it away from the mouth of the 
vent, so that although a deposit might be formed, it could not be in the form of 
a crater-bearing cone. 
The volcanoes of Ramii belong to a different class. There the mud is not 
produced by chemical means, but by mere mixture of shale and clay with water. 
The ejecting force is evidently the pressure of gas, which is in large part, at least. 
' Phil. Trans., 1873. 
5 In botli fusion and mud volcanoes mobility is conferred on a certain proportion of rock by 
mere breaking up sufficiently small to admit of ejection along with the fluid materials. 
® ‘ New Zealand, ’ pp. 401, 432. 
* SiUiman’s Journal, XXVI, 292, 
