<i 
920 PHYSIOGRAPHICAL GEOLOGY. —[Boox VII. 
may in most cases be correct. It is not so easy, however, to demon-. 
strate that a disturbance was rapid as to prove that it must have been 
slow. That some uplifts resulting in the rise of important mountain 
ranges have been almost insensibly brought about, can be shown 
from the operation of rivers in the regions affected, ‘Thus the rise 
of the Uinta Mountains has been so quiet, that the Green River, 
which flowed across the site of the range, has not been deflected, but 
has actually been able to deepen its canon as fast as the mountains 
have been pushed upward. The Pliocene accumulations along the 
southern flanks of the Himalayas show that the rivers still run in 
the same lines as they occupied before the last gigantic upheaval of 
the chain (p. 879). | 
2. Terrestrial Features due to Volcanic Action.—The 
two types of volcanic eruptions described in Book III. Part 1, give 
rise to two very distinct types of scenery. The ordinary volcanic 
vent leads to the piling up of a conical mass of erupted materials 
round the orifice. In its simplest form the cone is of small size and — 
has been formed by the discharges from a single funnel, like many of 
the tuff and cinder cones of Auvergne, the Eifel, and the Bay of 
Naples. Every degree of divergence from this simplicity may be 
traced, however, till we reach a colossal mountain like Etna, wherein, 
though the conical form is still retained, eruptions have proceeded 
from so many lateral vents that the main cone is loaded with minor 
voleanic hills. . Denudation as well as explosion comes into play ; 
deep and wide valleys, worn down the slopes, serve as channels for 
successive floods of lava or of water and volcanic mud. On the other 
hand the type of fissure-eruption in which the lava, instead of issuing 
from a central vent, has welled upward from many parallel or con- 
nected fissures, leads to the formation of wide lava-plains composed of 
successive level sheets of Java. By subsequent denudation these 
plains are trenched by valleys, and-along their margin are cut into 
escarpments with isolated blocks or outliers. They thus become great 
plateaux or table-lands like those of north-west Europe, the Deccan, 
and Abyssinia (pp. 256, 565). s 
The forms assumed by volcanic masses of older Tertiary and still 
earlier geological date are in the main due not to their original » 
contours, but to denudation. ‘Ihe rocks, being commonly harder 
than those among which they lie, stand out prominently, and often, 
in course of time and in virtue of their mode of weathering, assume — 
a conical form, which, however, has obviously no relation to that of 
the original volcano. Eminences formed after the type of the Henry 
Mountain (p. 546) owe their dome-shape to the subterranean effusion 
of erupted lava, but the superficial irregularities of contour in the 
domes must be ascribed to denudation. 
' Powell's Geology of the Uinta Mountains, in the Reports of U.S. Geographical — 
and Geological Survey, Ltocky Mountain Region, 1876. The same conclusion is drawn 
by Gilbert from the structure of the Wahsatch Mountains. See his admirable essay on 
“ Land Sculpture,” in his Geology of the Henry Mountains, published in same series of 
Reports, 1877, 
