THE ORIGIN OF IGNEOUS ROCKS. 
97 
Thus he concludes “ that particular rocks have no neces¬ 
sary relation to time on our globe.” * * * “ It requires 
only different circumstances as regards pressure, heat, and 
slowness of cooling to form any igneous rock the world con¬ 
tains.” 
In 1851 Bunsen,* after a visit to Iceland and a chemical 
investigation of the volcanic rocks occurring there, arrived 
at the conclusion that any of the varieties of these rocks 
could be considered as a mixture of two extreme magmas, 
one of which represented the most basic and the other the 
most acid form of the volcanic rocks of that region. His 
own calculations and comparisons show that he did not re¬ 
quire a very great degree of accord between the observed 
proportions of the various constituent oxides and those calcu¬ 
lated from his hypothetical extremes, even after he had com¬ 
bined the alumina and iron oxide, constituents which in the 
mineral composition of rocks have little or no connection 
with one another. He-suggests the theory that all of these 
volcanic rocks were derived from two reservoirs of magma, 
one the basic or “ normal pyroxenic ” and the other the acid 
or “ normal trachytic,” either separately or in combination, 
the two magmas commingling in various proportions to form 
the intermediate varieties. He seems to have viewed the 
question from a purely chemical standpoint, regardless of 
the geological or physical difficulties to be overcome. The 
simplicity of the hypothesis and the unquestionable chemical 
relationship between the volcanic rocks of this and of other 
regions, which was thus brought to light for the first time, 
gave a force to Bunsen’s theory that impressed it deeply upon 
the science, though for geological reasons it was very soon 
discredited. We are indebted to Bunsen, however, for call¬ 
ing attention to the chemical relationship of igneous rocks 
and initiating a line of investigation which is at present far 
from being completed. 
* Bunsen (R.) Ueber die Processe der vulkanischen Gesteinsbildun- 
gen Islands. Pogg. Ann. d. Phy. u. Chem. 8°. Leipzig, 1851, vol. 83, 
No. 6, pp. 197-272. 
16-Bull. Phil. Soc., Wash., Vol. 12. 
