122 
IDDINGS. 
selected by Rosenbusch he first points out a number which 
he thinks are of too greatly decomposed rocks to be service¬ 
able. He then shows that the sum of the metal atomic pro¬ 
portions is in no sense characteristic of any group of rocks. 
He follows the calculations of the ratios which Rosenbusch 
made out for the different rock magmas and shows that 
they are too widely divergent to establish the hypothetical 
“ kerns ” derived from them. He calls attention to the 
number of varieties of rocks embraced within each of these 
“ kerns,” trachyte appearing in three, and shows the difficulty 
of deriving the minerals of rocks from the hypothetical 
“ kerns.” 
In 1890, almost simultaneously with the article by Rosen- 
busch on the chemical relations of eruptive rocks, appeared 
a monograph by Brogger * on the minerals of the syenite- 
pegmatite dikes of southern Norway. As an introduction 
of this great work there is given an account of the geology 
of the region of Christiania with special reference to its 
eruptive rocks. This is but a preliminary sketch of what is 
eventually to be a monograph on these rocks—a book whose 
appearance will be looked for with the keenest interest. The 
igneous rocks of this region constitute a series of eruptions 
mostly of Devonian age, and form massive intrusions, dikes, 
and surface flows. There is a wide range of mineralogical 
and chemical composition and a well-defined order of erup¬ 
tion. The result of the investigation of these rocks has led 
Brogger to certain definite conclusions regarding their origin. 
The whole group of eruptions forms a continuous series of 
rocks which merge into one another petrographically and 
chemically. Commencing with very basic rocks, they be¬ 
come more and more silicious, until the last of the series, 
which are basic. The variation of the chemical constituents 
other than silica is different in different cases. These rela¬ 
tionships obtain for the coarsely granular rocks (Tiefenge- 
* Brogger (W. C.) “ Die Mineralien der Syenitpegmatitgange der Siid- 
norwegischen augit- und neplielinsyenite.” Zeitschr. fur Kryst u. Min., 
8°. Leipzig, 1890, vol. 16. 
