30! — 
The fishes are represented by remains of a fresh water shark, Pleura - 
cant hus, and of Atcgulichtus and Amply pterns . Some fossils indicate 
clearly a lower Permian age, probably the middle part of lower Perm- 
ian, while others are related to Carboniferous forms. 
Thus the fossils as well as the character of the sedimentary rocks 
point to a continental deposition in fresh water, with a general relation 
i 
to the Rotliegendes of Middle Europa. 
The thickness of the Permian sedimentary series varies much from 
north to south. Thus the lavas in the Brumunddal area are lyim* directly 
on weathered Silurian rocks, whereas around Oslo the series attains a 
thickness of 20—30 m. From south of Drammen to Horten only the 
quartz conglomerate is observed, and even this is lacking in some 
places. In the Skien district again Permian shales and sandstones are 
about 50 ni in thickness. 
The volcanic rocks. 
During the volcanic phase a thick series of volcanic rocks were 
formed over the entire Oslo region. The volcanics consist of trachy- 
andesitic lavas (so-called rhomb porphyries) and basaltic lavas, with 
a minor amount of acidic volcanics. The area covered by volcanics 
exceeded that of the present Oslo region (200 km X 40 km) and mav 
well have been 200 km X 100 km, possibly 300 km X 150 km. 
After erosion had carried off most of the rocks, the present volume is 
about 300 km 3 . The lava series may have had higher members which 
are now removed and the total volume may have been as much as 
6000 km 3 or more (Oftedahl i 952 ) . 
The eruptions may be divided into three phases. The first phase 
of volcanism consisted in the formation of a lava plateau comprising 
between 10 and 20 single flows of great horizontal extension. These 
effusives are clearly fissure eruptions, as early pointed out by Brogger. 
The second phase is characterized by a number of volcanic centers with 
large basalt volcanoes with minor amounts of acidic (ignimbritic) 
rocks and by explosive centers associated with similar acidic rocks. 
During the third phase the volcanic activity returned to fissure erup 
tion type. 
The detailed stratigraphy of the lava flows was first worked out 
for the Krokskogen lava plateau by Brogger, Schetelig and collabora- 
tors in the period 1910 — 17. This proved possible because the different 
