CHAP. V 
ERUPTIVE VENTS 
53 
and in former geological periods, such differences exist between the materials 
ejected from vents not far distant from each other as to show the existence 
of more limited distinct reservoirs of liquid rock underneath. 
Home of the questions here asked will be further dealt with in later 
pages in connection with such geological evidence as can be produced 
regarding them. But it will be found that at every step in the endeavour 
to ascertain the origin of volcanic phenomena difficulties present themselves 
which are now and may long remain insoluble. 
I. Texts of Euuptjon 
It is a general belief that the first stage in the formation of a \’olcano 
of the Vesuvian type by the efforts of subterranean energy is the rending of 
the terrestrial crust in a line of fissure. Some of the most remarkable 
groups of active volcanoes on the face of the globe are certainly placed in 
rows, as if they had risen along some such great rents. Tlie actual fissure, 
however, is not there seen, and its existence is only a matter of pi'obable 
inference. Undoubtedly the effect of successive eruptions must be. to con- 
ceal the fissure, even if it ever revealed itself at the surface. 
What is supposed to have marked the initial step in the formation of a 
great volcano is occasionally repeated in the sulisequent history of the moun- 
tain. During the convulsive shocks that precede and accompany an erup- 
tion, the sides of the cone, and even sometimes part of the ground Iteyond, 
are rent open, occasionally for a distance of several miles, and on the fissures 
thus formed minor volcanoes are built up. 
It is in Iceland, as already stated, that the phenomena of fissures are 
best displayed. There the great deserts of lava are from time to time 
dislocated by new lines of rent, which ascend up to the surface and stretch 
for horizontal distances of many miles. From these long narrow chasms 
lava flows out to either side ; while cones of slag and scoria? usually form 
upon them. Tliis interesting eruptive phase will be more fully described 
in the chapters dealing with the Tertiary volcanic rocks of Britain. 
There can be no doubt, however, that in a vast number of volcanic vents 
of all geological periods no trace can be discovered of their connection with 
any fissure in the earth’s crust. Such fissures may indeed exist underneath, 
and may have served as passages for the ascent of lava to within a greater 
or less distance from the surface. But it is certain that volcanic energy has 
the power of blowing out an opening for itself through the upper part of the 
crust without the existence of any visible fissure there. Wliat may be the 
limits of depth at which this mode of communication with the outer air is 
possible we do not yet know. They must obviously vary greatly according 
to the structure of the terrestrial crust on the one liand, and the amount 
and persistence of volcanic energy on the other. We may suppose that 
where a fi.ssure terminates upward under a great depth of overlying rock, 
the internal magma may rise up to the end of the rent, and even be injected 
laterally into the surrounding parts of the crust, but may be unable to com- 
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