OF VOLCANIC KOCKS. 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 volcano 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 by the 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 lava, vapors and boiling water, and other phenom- 

 ena associated with volcanic 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 volcanic 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 case, 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 (123) 



