292 NATURAL SCIENCE. 



JUNE, 



Palaeozoic formations is thus over i,ooo metres. The oldest igneous 

 rocks are interstratified with the uppermost members of the Palaeozoic 

 sequence, and, therefore, probably belong to the Old Red Sandstone 

 period. They are basic rocks of the diabase family, containing from 45 

 to 47 per cent, of silica. The plutonic rocks of the same age and 

 character occur as small but well-characterised laccolites at Solvsberg 

 and Brandbokampen on the east side of the Randsfjord. The rock 

 of the former locality is an olivine-diabase of granitic structure 

 (gabbro-diabase), composed of plagioclase, olivine, violet pyroxene, 

 biotite, and iron ores ; that of the latter a coarse-grained proterobase 

 (gabbro-proterobase) containing a basic plagioclase, pyroxene, 

 brown basaltic hornblende, and biotite, together with idiomorphic 

 titanite, magnetite, and pyrite. An apophysis from the proterobase of 

 Brandbokampen shows a marked porphyritic structure. The pheno- 

 crysts are formed of brown hornblende and pyroxene ; the ground- 

 mass of lath-shaped plagioclase, prisms of brown hornblende, and iron 

 ores. This rock is regarded by the author as a typical camptonite. 

 The area occupied by intrusive basic rocks is, however, very small 

 compared with that covered by the sheets. The rocks of these sheets 

 are variable in character, and are grouped under the terms augite- 

 porphyrite, melaphyre and labrador-porphyrite. 



The plutonic rocks of the second group include basic augite- 

 syenites (laurvikite), mica-syenites, -and nepheline-syenites (laurda- 

 lite). They are especially developed in the south-western portion 

 of the district, and form the coast-line between Tonsberg and Lange- 

 sund. The augite-syenite is a coarse-grained rock with a peculiar 

 structure due to the abundance of a felspar which occurs in large 

 individuals, and shows a marked tendency to idiomorphism, with 

 development of the forms |iio| and {201}. These forms give rhombic 

 sections in almost all directions. The other constituents occur in 

 smaller individuals, and are wedged in between the conspicuous 

 felspars. The constituents are soda-orthoclase or soda-microcline 

 (anorthoclase), violet (titaniferous) or green (alkali-bearing) pyroxene, 

 biotite, brown hornblende, olivine (usually but not invariably present), 

 titaniferous magnetite or ilmenite, apatite, nepheline, and sodalite 

 (both common, but only present in small quantity). For this special 

 type of augite-syenite, which is distinguished from other types by 

 the character of the felspar and the presence of nepheline and 

 sodalite as accessory constituents, Brogger proposes the name of 

 laurvikite. 



The mica-syenites are closely related to the above, but the felspar 

 is mostly oligoclase, and gives rectangular instead of rhombic sections. 

 Biotite, as the name implies, is abundant. 



The nepheline-syenites have the same structure, and are com- 

 posed of the same minerals as laurvikite, but nepheline (elaeolite) 

 occurs as an essential constituent, and sodalite is often present. The 

 texture is very coarse, and the individuals of nepheline are sometimes 



