4o6 PETROLOGICAL ABSTRACTS AND REVIEWS 



In the first group are lavas, tuffs, agglomerates, and dike-rocks. Pillow- 

 lavas are common. The intrusives of this group are gabbros and olivine-rich 

 rocks. These are often saussuritized, uralitized, or converted into serpentine. 

 Analyses of members of this class, as well as of the two others, are presented. 

 The green lavas probably range in time from the upper part of the Ordovician 

 into the Silurian; they are apparently homologues of the greenstones of Eng- 

 land and Wales, which, however, were extruded at a slightly earlier date. 



The anorthosite-charnokite rocks vary greatly in character. Some are 

 typical gabbros and norites, others bear a little potash feldspar. Uralitiza- 

 tion is common here. 



Masses of pyroxenites and peridotites also belong to this period; these 

 generally appear as lenses enclosed in basic or intermediate masses of more 

 recently consolidated magmas of the same group. Another type of rock found 

 here is labradorite-rich, hypersthene-bearing "labradorfels" ( = anorthosite) . 

 All the above rocks of this segregation group are basic. There are also norites 

 and mangerites; hypersthene-syenites represent a gradation toward the more 

 alkalic end of the series; and finally acid rocks are represented by granites — 

 hypersthene-granite (birkremite), commonly showing droplet-like plagioclase 

 within the potash feldspar, augite-granite, diopside-granite, aegirite-granite, 

 amphibole-granite, and biotite-granite, the latter two forming the commonest 

 acid rocks of the group. Pegmatites and dike-rocks are common, and the latter 

 range from acid to basic (peridotites). The sequence in the anorthosite-charno- 

 kite group is known with certainty to be (i) pyroxenite-peridotite, gabbro and 

 norite, (2) norite, mangerite, and labradorite, (3) pyroxene-syenite, monzo- 

 nite, and granite. 



These anorthosite-charnokite rocks appear to have played a noteworthy 

 part in the diastrophism of the region. They are assigned to early Caledonian 

 age. 



The third group studied comprises pyroxenites, peridotites, and gabbros; 

 intermediate rocks, too, such as diorites, hypersthene-mica-diorites, and a 

 pecuHar type newly called "opdalite," which appears to be essentially a quartz- 

 diorite. The acid phase is represented by "trondhjemite," (new name) which 

 Kjerulf calls an oligoclase-granite, and which might as well be characterized 

 as a granodiorite. Associated with these rocks are some dike-rocks, which 

 incline generally toward the acidic; thus trondhjemite-porphyries, -apHtes, and 

 -pegmatites are recognized. The porphyritic phases occur already as borders 

 of the larger intrusive masses. 



A noteworthy fact is the sharp boundary of these aplites against the schistose 

 basic country rock. This is explained by applying the principles of differentia- 

 tion through fractional crystallization. A granite which has differentiated and 

 achieved stability through having the basic constituents of the mother magma 

 crystallize out, cannot resorb similar basic components; a magma can only 

 assimilate the country rock when the mineral constituents of the latter have 

 not been previously lost by differentiation in the mother magma. 



