OF VOLCANIC ROCKS. 85 
the configuration of their surface which cannot be more plausibly explained in any 
other way. To these belong the sudden breaks in the continuity of their surface, 
which consist sometimes in elongated and steep walls, thousands of feet in height, and 
miles in length, or in crateriform or semi-circular basins, and in other more or less 
abrupt depressions, which may be chiefly noticed where large regions are uniformly 
covered with granite, porphyry or volcanic rocks. This cause will affect the mass of 
the eruptive rock itself, but not perceptibly the surrounding country. Yet, this is 
subsiding when a voleano is active, and it can be definitely proved in the case of the 
andesitic ranges of Hungary, that during the epochs of the massive eruptions, the gen- 
eral rise has been repeatedly interrupted bythe subsidence of the suroundings of the 
theater of activity. To these changes applies probably the second of the causes alluded 
to. Whenever a fissure is filled by liquid matter injected from below, its surroundings 
must necessarily become heated, and, by their expansion, produce a slight increase of 
the rise of the surface. This heat escapes, in the case of a volcano, chiefly in the epochs 
of its activity, by the emission of laya, vapors and boiling water, and other phenom- 
ena associated with voleanic action. Massive eruptions will have been attended by the 
escape of heat on a much larger scale. They appear to have been often accompanied 
by an extremely violent emission of hot water, as may be inferred from the great 
accumulation of deposits of silica in some volcanic countries, or from the immense 
overflows of extensive regions by voleanic mud: this occurs in Hungary and on the 
western slope of the Sierra Nevada on so grand a scale as to almost exclude the possi- 
bility of its having originated merely from volcanic action, especially as no volcano is 
visible from which they could have escaped. These processes must of course have had 
the effect of lowering the temperature of the masses surrounding the fissure to some 
distance from it, and of producing subsidence during the eras of activity. Yet they 
are not sufficient to explain the extent to which it has often taken place, though it had 
in no ease, in the vicinity of the theaters of eruptive action, more effect than to reduce 
locally the amount of elevation. 
A few examples will suffice to show how vast are the changes of level which 
have taken place since the commencement of the volcanic era, and to demonstrate their 
connection with the other manifestations of vulcanism during the same era. An in- 
structive instance is furnished by the country situated between the Pacific coast and 
the Rocky Mountains. The labors of several distinguished geologists have made us 
acquainted with some of the main features of its complicated structure ; but it is only 
by the detailed examination of some of the most important portions, together with the 
accurate determination of the age of several of the sedimentary and metamorphic forma- 
tions, made by and under the direction of Professor Whitney, that the foundation has 
been laid for an exact exploration of the entire western part of North America, which is 
now proceeding with rapid steps, and promises to give important contributions towards 
the solution of the questions discussed in this essay. There appears to have existed, as 
we mentioned before, an ancient granitic era in that region. But it is as yet impos- 
sible to recognize the relation of this ancient granite and the metamorphic rocks by 
which it is accompanied to the ancient or to the present configuration of the surface. 
In the western countries, those formations are concealed by the immense overlying 
P (125) 
