THE ORIGIN OP IGNEOUS ROCKS. 
105 
facies of one body of igneous rock by the separation (spal- 
tung) of the original mass into different mineral associations ; 
but he did not apply this principle to the development of 
certain varieties of rocks from some pre-existing magma, as 
Duroeher did, nor did he consider the general question of 
the origin of igneous rocks. The same may be said of his 
more recent publications in 1869, 1887, and 1891, to which 
reference will be made in another connection. 
In 1862 Jukes,* after referring to Bunsen’s and Duroeher’s 
theories, remarks : “The identity or very great similarity of 
the various volcanic products in all parts of the world seems 
to point to a common origin for them. The frequent asso¬ 
ciation in all parts of the earth of the two great classes of 
these products, the trachytic or purely feldspathic (or highly 
silicious, with little alkali, lime, or iron) and those in which 
the feldspathic are largely mingled with hornblendic or 
augitic minerals (containing much alkali, lime, and iron), 
seems to me to show that their separation is not so much due 
to diversity of origin as to some cause tending to segregate the 
one from the other out of a generally diffused mass in which 
the constituents of both may be equally mingled. 
“ If we assume all igneous rocks to proceed either from one 
central molten mass of equable constitution throughout or 
from separately fused portions of perfectly similar constitu¬ 
tion, might we not suppose that the difference in the consti¬ 
tution of the various products which we find at the surface 
depended on the circumstances and conditions in which they 
had been placed ? The portions now open to our examina¬ 
tion had probably to pass through different conditions of 
temperature and pressure, which might perhaps alone cause 
a separation to take place in their different ingredients. 
They might also take up in their passage other ingredients, 
or more of some of those they already contained.” He also 
suggests that at times when violent accessions of heat ap- 
* Jukes (J. B.) The Student’s Manual of Geology. 8°. Edinburgh, 
1862, pp. 340-341. 
17—Bull. Phil. Soc., Wash., Vol. 12. 
