
STRUCTURE AND SURFACE FEATURES 403 
type are obviously due to the bulging-up of the crust over a 
concealed mass of molten matter (Fig. 66, p. 191). JLaccolith 
mountains may formerly have been conspicuous in our own 
and other lands, where intrusive igneous rocks abound. 
Many boss-like masses and thick lenticular sheets of basalt 
and other rocks form conspicuous features in the Scottish 
lowlands (see pp. 191-195). These at the time of their 
intrusion were covered more or less deeply with sedimentary 
strata which they could not pierce, but may well have lifted 
up so as to cause prominent dome-shaped bulgings at the 
surface. But in the case of such ancient igneous rocks, so 
long a time has elapsed since their intrusion that any super- 
ficial bulging they may have caused has entirely disappeared. 
Recognisable laccolith mountains are necessarily of recent 
formation. 
Erosion of Tectonic Mountains and Resulting Features.—Some 
tectonic mountains date back to a most remote geological antiquity, 
while others are young—not a few having come into existence in 
relatively recent times. In the case of the youngest mountains of this great 
class, internal structure, as might have been expected, not infrequently 
coincides more or less closely with external form or configuration. This 
correspondence is most clearly seen in recent Accumulation mountains, 
such as our still active volcanoes—the shape assumed by those mountains 
being obviously determined by the arrangement of their constituent 
materials. Nevertheless, even active volcanoes do not escape the 
modifying influence of the various epigene agents of change, but are 
attacked in the same way as mountains of every kind, old and young 
alike. In their case, however, the rate of decay is usually exceeded by 
the rate of growth. Hence the rugged furrows and gorges, gouged out 
by torrents on the flanks of a growing volcano, tend to be obliterated 
from time to time by the products of successive eruptions. But the great 
chains and ranges of Folded mountains cannot thus repair the ravages 
effected by epigene action. The growth of mountains of this type, we 
have every reason to believe, is a very gradual and protracted process. 
No sooner, therefore, does upswelling and wrinkling of the crust begin, 
than the slowly ascending surface is attacked by all the atmospheric 
agents of change. And so powerful and effective is this action, that if 
the rate of crustal movement did not exceed the rate of denudation, no 
mountain range could come into existence. It would be degraded as 
fast as it grew. Obviously, however, crustal deformation, no matter how 
gradual it may be, has in many, if not in all, cases exceeded the rate of 
denudation. Nevertheless, so potent are the agents of erosion that they 
have succeeded in very greatly modifying even the youngest elevations 
of the crust. 
Although no portion of a growing mountain chain can escape this 
