1895.] Petrograp hy. 567 
PETROGRAPHY.' 
The Eruptive Rocks of the Christiana Region.—Brigger 
has done an excellent piece of work in this, the first of his reports on 
the eruptive rocks of Norway. The article deserves much more notice 
than can be given it in this place. Briefly, the author describes groru- 
dite, salvsbergite and tinguite dykes whieh together form what is de- 
nominated a rock series—that is, a series of rocks that differ slightly 
from each other in their chemical composition, but which, at the same 
time, by their intimate gradations into each other, give evidence of 
being closely related. All of these rocks are rich in soda and potassa, 
and all contain alkaline amphiboloids. The grorudite is essentially an 
aggregate of microcline and albite, usually in microperthitic inter- 
growths, rarely anorthoclase, and always aegerine and amphibole, as 
phenocysts, in a groundmass of potash feldspar, albite, sometimes soda- 
orthoclase, aegerine and more or less quartz. The amphiboles are 
arfvedsonite and katoforite, the latter name being given to a series of 
alkaline iron amphiboles having the angle C A ¢ = 31°-58°, and 
pleochroism as follows: B>C> A= yellowish red > brownish red 
> yellowish red or greenish yellow. In all their properties, so far as 
studied, they occupy a position between barkevikite and arfvedsonite. 
Salvsbergite differs from grorudite in containing little or no quartz. 
Its structure is trachytic. 
Grorudite is regarded as the dyke form of soda-granite and panteller- 
ite and salvsbergite that of nordmarkite. 
After a discussion of the significance of the notion of dyke rocks as 
a group of well-defined rock types, the author concludes that while the 
group is well characterized by Rosenbusch, it includes a number of 
rocks that are but apophyses of bosses, etc., and which should be classed 
with the rocks of bosses. He prefers the term “hypabyssische Ges- 
teine” for all rocks with the structure of dyke rocks, whether they be 
in the form of true dykes, of sheets, or whether they occur as the peri- 
pheral form of bosses or laccolites. The hypabyssmal rocks comprise a 
great group of equal value with that of the surface (volcanic) rocks 
and that of the abyssmal (plutonic) rocks. It includes two classes— 
the aschistic and the diaschistic—the first embracing those rocks not 
produced by the differentiation of their source-magma, and the latter 
1 Edited by Dr. W. S. Bayley, Colby University, Waterville, Maine. 
2 Viedenskabsselskabels Skrifter. Math.-naturv. Klasse, 1894, No. 4. 
