100 
IDDINGS. 
In 1855 Lyell * * discusses the supposed connection between 
the composition of igneous rocks and the age of their erup¬ 
tion, the most silicious being always considered the oldest, 
and remarks that “ the hypothesis suggested by Mr. Scrope 
may perhaps afford a solution of this problem.” But the 
explanation which he proceeds to give was not derived from 
the suggestions of Scrope already quoted, as will be seen by 
the following: “ If, therefore,” says Lyell, “ a large quantity 
of rock be melted up in the bowels of the earth by volcanic 
heat, the denser ingredients of the boiling fluid may sink to 
the bottom, and the lighter, remaining above, would in that 
case be first propelled upward to the surface by the expan- . 
sive power of gases. Those materials, therefore, which occupy 
the lowest place in the subterranean reservoir will always be 
emitted last and take the uppermost place on the exterior of 
the earth’s crust.” This he repeated in 1871.f 
This is a separation by specific gravity, which was sug¬ 
gested by Darwin and modified by Dana, Scrope’s theoiy 
being one of vaporization and squeezing out. 
Two years later, in 1857, Jukes % argues against the 
idea that there was a relation between the mineral compo¬ 
sition of igneous rocks and their geological age—a view 
also combatted by Scrope, hut advocated by von Walters- 
hausen and Lyell. 
Jukes would connect the composition with space or lo¬ 
cality ; thus he thinks “ that the proper quantity of silica is 
referable to the depth at which an igneous rock has been 
Gesteinsnuancen hervorgegangen sind.” * * * “ Als eine blosse Fic¬ 
tion konnte man diese Hypothese wolil hinnehmen, nur muss man nicht 
glauben, dass sie in der Natur begriindet sei, oder dass wirklich im Innern 
der Yulkane zwei gesonderte Herde mit basischen und sauren Fliissig- 
keiten vorhanden waren.” 
* Lyell (C.) A Manual of Elementary Geology. 8°. New York, 1855, 
p. 522. 
f Lyell (C-) The Student’s Elements of Geology. 8°. London, 1871, 
p. 508. 
t Jukes (J. B.) The Student’s Manual of Geology. 8°. Edinburgh, 
1857, p. 81. 
