THE ORIGIN OF IGNEOUS ROCKS. 
123 
steine), the extrusive rocks, and to a great extent for the dike 
rocks. 
The correspondence between the petrographical and geo¬ 
logical sequences appears to Brogger to prove undoubtedly 
a genetic connection between successive eruptions; and this, 
he thinks, is substantiated by the fact that the greater part 
of the rocks occurring in this region are not found in any 
other part of the world, and that many of those which do 
occur elsewhere have an entirely different habit in the region 
of Christiania. Moreover, there are many rocks of common 
occurrence in other regions which are not found in the one 
just mentioned. The greater part of the series is character¬ 
ized by a high percentage of soda. These relationships in¬ 
dicate that the successive eruptions must have originated 
from a common, separate, and enclosed reservoir of magma 
(“ Magma basin ”), which cannot be simply a portion of a 
universal fluid magma forming the interior of the-earth. 
He adopts Soret’s principle as the probable cause of the 
differentiation of the magma, which is considered to be a 
solution of silicates, and suggests that possibly a partial crys¬ 
tallization may have set in at the same time, producing 
segregations of the more basic minerals. 
A diffusion of the less soluble constituents having taken 
place toward the cooler margins of the reservoir, the first 
eruptions will be the most basic, and will be followed by 
more and more silicious ones. The final eruption of basic 
magma is possibly due to the accumulation at the bottom 
of the reservoir of crystals of basic minerals that have crys¬ 
tallized and settled, and have been melted again. The 
character of the whole series of rocks will depend on the 
composition of the original magma, which in this region, as 
already remarked, was rather rich in soda. The origin of 
the reservoir of molten magma is not discussed. 
In the following year, 1891, Vogt * published a paper on 
* Vogt (J. H. L.) “ Om Dannelsen af de vigtigste in Norge og Sverige 
representerede grnpper af jernmalmforekomster. Geol. Foren. i Stock¬ 
holm Forhand.” 8°. Stockholm, 1891, May, Yol. 13, p. 476. Reviewed 
by J. J. H. Teall in the Geological Magazine, Feb. 1892. 
