106 
IDDINGS. 
proached the surface trachyte and felstones might be poured 
out; at other times more basic rocks. 
In these expressions he refers all igneous rocks to one 
common magma, but does not suggest any hut the vaguest 
causes for their differentiation. 
In 1866 Zirkel* reviewed at considerable length the theo¬ 
ries of Bunsen, Yon Waltershausen, Yon Cotta, Durocher, and 
the methods of classification of Kjerulf, Roth, and Scheerer, 
without advancing any particular hypothesis of his own 
with regard to the origin of igneous rocks. He appears to 
adopt that of Von Waltershausen as the most satisfactory. 
In 1868 Yon Richthofen, in his celebrated paper on 
“ The Natural System of Volcanic Rocks,” f discusses at 
great length the origin of volcanic rocks. He attempts to 
establish a distinction between the products of volcanic 
action and massive eruptions, and states that “ volcanic 
action and massive eruptions, notwithstanding the similar¬ 
ity of the material produced by both, would appear * * * 
to differ to some extent, not only in regard to the causes to 
which they owe their origin, but also in regard to the posi¬ 
tion the matter occupied before its ejection.” He observes 
that the law of Bunsen is true for all eruptive rocks, but this 
has been abundantly disproven by Roth and others. 
He considers that “ the source of volcanic rocks has been 
similar in nature in every locality; that the definite numeri¬ 
cal relations must exist at that source, and that at the same 
source the different kinds of matter which correspond to the 
different passages in composition among volcanic rocks must 
be arranged, at every locality alike, in a definite order of 
position vertically, in order to explain the order of succession 
of eruption.” He observes that the arrangement of layers 
of increasing specific gravity within the earth was first stated 
by Yon Waltershausen. 
* Zirkel (F.) Lehrbuch der Petrographie. 8°. Bonn., 1866, vol. 1, pp. 
453-473. 
|Von Richthofen (F.) The Natural System of Volcanic Rocks. 4°. 
San Francisco, 1868. 
