THE ORIGIN OF IGNEOUS ROCKS. 
93 
that there was no evidence of repulsion between these two 
rocks, “ the production of trachyte preventing that of basalt, 
and vice versa; ” but “ the general law seems to be that they 
occur together, being produced successively from the same 
or proximate vents, though, in general, at long intervals of 
time.” Scrope thus advocated the view that the various 
lavas originated from a common source, which was a pre¬ 
existing crystalline rock, by a process of differentiation caused 
by the volatilization of the ferruginous minerals and their 
separation in a gaseous state from the feldspathic portion by 
pressure. 
In 1844 Charles Darwin * published his observations on 
Volcanic Islands, and expressed his views as to the origin 
of igneous rocks. He appears to have been led to his posi¬ 
tion by observing in certain instances that the porphyritical 
crystals in some lavas have settled to the bottom of the lava- 
sheet, and he cites the case of certain obsidians in which this 
is said to have taken place. He therefore considers the con¬ 
sequence of the partial crystallization of minerals and their 
settling because of their specific gravity, and suggests that 
this explains why, as he believes, basalts generally break out 
at the base of volcanoes and obsidians from their summits. 
In applying this theory to plutonic rocks he considers green¬ 
stones and Jsasalts as having separated out from earlier rocks, 
and mentions the occurrence of dikes of these rocks in granite 
regions, and asks the question, Whence have these dikes 
come ? Are we to suppose a substratum of trap beneath the 
granitic series ? “ Is it not more probable that these dikes 
have been formed by fissures penetrating into partially cooled 
rocks of the granitic and metamorphic series, and by their 
more fluid parts, consisting chiefly of hornblende, oozing out 
and being sucked into such fissures ? ” As an illustration of 
this, he describes an instance in which he observed thin 
threads of dark rock penetrating the gneiss on either side of 
dikes of a dark-colored augitic or hornblendic rock, and 
states that he considers these to have been the “ feeders ” of 
* Darwin (C.) Volcanic Islands. 8°. London, 1844, pp. 117-124. 
