J 826.] Mr.ScropeonVokanos. 53 



a time, and forced at length to break out by another issue ; so 

 the accumulation of similar obstructions, of greater magnitude, 

 and during a longer period of eruption, checks the general 

 expansive force of the subterranean caloric for intervals of great 

 duration, and drives it to force an issue at length in some other 

 direction, perhaps at a superficial distance of some hundreds of 

 miles, leaving the former vent to ail appearance extinct. 



The jarring and vibratory motion occasioned by the sudden 

 rupture of the overlying mass, is supposed to occasion the pheno- 

 mena of earthquakes, vhich are felt over a greater or less surface 

 according as the seat of expansion is confined to the focus of 

 an habitual volcano, or exists at a greater depth, and extends 

 more widely. 



To repeated expansions of this nature taking place at a great 

 depth, Mr. Poulett Scrope attributes the elevation of mountain 

 chains ; the fractures, contortions, and other irregularities of 

 their strata resulting from the immense friction occasioned by 

 the protrusion through them of the crystalline rocks in a solid 

 state (granites, porphyries, serpentines, &c.), and the subsidences 

 produced by their elevation to high angles while often in a semi- 

 solid state. 



But as it is not on these lines of greatest elevation that vol- 

 canos have usually burst forth, the author conceives this process 

 to occasion the formation of parallel fissures at a distance, by 

 the lateral drag accompanying the forcible elevation of the 

 superficial strata; and that the sudden reduction of pressure on 

 the internal mass of heated rock immediately under these clefts 

 or the largest and deepest of them, causes it to intumesce and 

 break out on various points; thus giving rise to those linear 

 chains of volcanic vents which are so remarkably obvious on the 

 globe's surface, and which frequently present a decided parallel- 

 ism to one another, and to some neighbouring rano-e of conti- 

 nental mountains. The activity of these lateral vents propor- 

 tionately retards the increase of expansive force general to the 

 subterranean rock, and tends to check the farther elevation of 

 the neighbouring continent. Hence volcanos act as sajetu- 

 valves to the globe; while in their origin they are only secondary 

 and attendant circumstances to the more immediate and primary 

 result (as well as Jinal cause) of subterranean expansion, viz. 

 partial elevations of the solid crust of our planet. 



This theory is then applied to account for the most remarkable 

 features in the surfaces of continents ; and the autlior goes on 

 to examine, though in a more cursory way, as not properly 

 belonging to his subject, the probable origin of the various 

 stratified rocks, and the causes of their distinguishing charac- 

 ters. 



This leads him to hazard a sketch of a theory of the globe. 



