THE ORIGIN OF IGNEOUS ROCKS. 
91 
HISTORICAL REVIEW. 
Among the earliest views regarding the nature and origin 
of volcanic rocks the most prominent conceived the various 
molten lavas to have existed as such from the first consoli¬ 
dation of the earth—that is, to belong to the originally fused 
matter of the globe—while another class of theories con¬ 
sidered them to be fused portions of metamorphic or sedi¬ 
mentary rocks. Without entering into the earliest of these 
theories, we may begin our review by mentioning the first 
expressions of Scrope * on the variations of volcanic rocks, 
made in 1825, in his work on “Volcanos.” It is to be re¬ 
marked that at that time Scrope considered volcanic rocks 
or lavas as erupted forms of previously crystallized rocks, 
which were rendered fluid by “ intumescence,” the cracks 
between the mineral crystals being filled with water which, 
when highly heated, gave fluidity to the whole mass. The 
rocks themselves were not considered to have been actually 
molten in most cases at the time of eruption. 
This explanation is necessary to an understanding of his 
views regarding “ the circumstances which may have given 
rise to the variety of mineral composition observable in the 
lavas.” 
“ This variety,” he says, “ must be supposed, owing to one 
or* other of two causes, viz: 
1. “An original difference in the nature of the subter¬ 
ranean crystalline rock, whose extravasation produced them 
on the surface of the earth; or, 
2. “To changes produced in this rock, originally of an 
uniform composition/during the process of elevation, which 
w T as probably accompanied in many instances by repeated 
intumescence and reconsolidation before the final emission 
of the lava. 
“ I own that I incline to the latter alternative, although I 
am far from being able to explain the mode in which every 
such change can have taken place. 
* Scrope (G. P.) Volcanos. 8°. London, 1825, pp. 21, 25, 26. 
