92 
ELEMENTS OF GEOLOGY. 
clinal and synclinal axes before alluded to, and there is no 
doubt that the subterranean causes of both forms of disturb¬ 
ance are to a great extent the same. A fault in Virginia, 
believed to imply a displacement of several thousand feet, 
has been traced for more than eighty miles in the same di¬ 
rection as the foldings of the Appalachian chain.* An hy¬ 
pothesis which attributes such a change of position to a suc¬ 
cession of movements, is far preferable to any theory which 
assumes each fault to have been accomplished by a single 
upcast or downthrow of several thousand feet. For we 
know that there are operations now in progress, at great 
depths in the interior of the earth, by which both large and 
small tracts of ground are made to rise above and sink be¬ 
low their former level, some slowly and insensibly, others 
suddenly and by starts, a few feet or yards at a time; where¬ 
as there are no grounds for believing that, during the last 
3000 years at least, any regions have been either upheaved 
or depressed, at a single stroke, to the amount of several 
hundred, much less several thousand feet. 
It is certainly not easy to understand how in the subterra¬ 
nean regions one mass of solid rock should have been folded 
np by a continued series of movements, while another mass 
in contact, or only separated by a line of fissure, has re¬ 
mained stationary or has perhaps subsided. But every vol¬ 
cano, by the intermittent action of the steam, gases, and 
lava evolved during an eruption, helps us to form some idea 
of the manner in which such operations take place. For 
eruptions are repeated at uncertain intervals throughout the 
whole or a large part of a geological period, some of the sur¬ 
rounding' and contiguous districts remaining quite undis¬ 
turbed. And in most of the instances with which we are 
best acquainted the emission of lava, scoria, and steam is 
accompanied by the uplifting of the solid crust. Thus in 
Vesuvius, Etna, the Madeiras, the Canary Islands, and the 
Azores there is evidence of marine deposits of recent and 
tertiary date having been elevated to the height of a thou¬ 
sand feet, and sometimes more, since the commencement of 
the volcanic explosions. There is, moreover, a general ten¬ 
dency in contemporaneous volcanic vents to affect a linear 
arrangement, extending in some instances, as in the Andes 
or the Indian Archipelago, to distances equalling half the 
circumference of the globe. Where volcanic heat, there¬ 
fore, operates at such a depth as not to obtain vent at the 
surface, in the form of an eruption, it may nevertheless be 
conceived to give rise to upheavals, foldings, and faults in 
* H. D. Kogers, Geol. of Pennsylvania, p. 897. 
