ABSAROKITE-SHOSHONITE-BANAKITE SERIES. 955 
In his account of the rocks of the grorudite-tinguaite series,’ 
Professor Brogger describes a series of igneous rocks occurring 
as dikes that are characterized by mineralogical and chemical 
peculiarities, which distinguish them from all other rocks in the 
Christiania region, and which warrant their being separated into 
distinct classes. These have been named grorudite and sdlvs- 
bergite, the third member of the series being tinguaite. Between 
these classes are transitional varieties. The rocks of this series 
grade through mineralogical and chemical transitions into other 
rocks belonging to different series, such as laurvikite, laurdalite, 
nordmarkite-and soda-granite. They represent a series of par- 
ticular phases of differentiation of genetically related magmas, 
as in the case of the series described from the Yellowstone Park. 
The chemical composition of the rocks in the grorudite- 
tinguaite series, as given by Brégger, is shown in Table VI. The 
whole series is more siliceous than those we have been consider- 
ing, the range of silica being from 56.58 to 74.35 per cent. 
Some of the marked characteristics of this series are the strong 
decrease in alumina with increase of silica; also the correspond- 
ing increase in ferric oxide; the relatively high percentage of 
alkalies which increase with alumina, soda being greater than 
potash. Magnesia and lime are very low. The mineral char- 
acteristics are equally marked. 
The consistent minerals are potash-feldspar and soda-feldspar, 
with egerite (or alkaline hornblende) with quartz in some cases, 
without it in others, and with nepheline in others. Plagioclase 
is entirely wanting, and mica and hornblende are generally 
scarce. The chemical character of the series is distinctly dif- 
ferent from that of the series of rocks just described from Yel- 
lowstone Park and Montana. A comparison of these series 
shows to what extent the products of differentiation in various 
regions may differ from one another, the laws of variation being 
quite opposite sometimes. Hence natural series of rocks, which 
may be genetically related, may traverse any scheme of sys- 
t Brogger, W. C. Die Eruptivgesteine des Kristianiagebietes. 1. Die Gesteine 
der Grorudit-Tinguaite-Serie. Kristiania, 1894. 
