210 
to their macroscopic appearance the rocks of the volcanic 
series have been mentioned in the geological chapter by the 
names of diabases, porphyrites, porphyries, and quartz-porphyries. 
From each of these groups a typical representative has been 
selected for description. 
DIABASE (TRACHYDOLERITE) OF NUNASARNAUSAK. 
The diabase which forms the flat top of the Nunasarnau- 
sak mountain is the relic of an intrusive sheet in the sand- 
stone. The thickness of the sheet is about 150 meters. The 
rock has an ordinary dolerite- or diabase-like appearance. Its 
colour isa greenish black or grayish black and its structure is 
ophitic and more or less fine-grained, the tabular felspar- 
crystals having an average length of about one millimeter. At 
the lower surface of the sheet as well as at the summit of the 
mountain the structure is almost dense and sometimes porphyr- 
itic; in some places a flow-structure is also seen here. 
Microscopic characters. — Examined in thin slices under 
the microscope the rock proves greatly altered. The alterations 
are probably due to contact-metamorphism and in many respects 
recall those of the Tertiary basaltic lavas of Skye, as described 
by Harker’. It can be established that the following minerals 
have constituted original components of the rock: apatite, iron- 
ore, olivine, augite, and plagioclase (labradorite). Judging from 
the chemical composition of the rock orthoclase and perhaps 
nepheline, brown hornblende, and mica have probably also been 
originally present, but the microscopical examination gives no 
downright proofs of this. 
The apatite is comparatively abundant in slender prisms or 
needles. The iron-ore is in crystals and irregular grains, it 
probably consists of titano-magnetite. 
1 A. Harker, The Tertiary Igneous Rocks of Skye. Memoirs of the Geo- 
logical Society of the United Kingdom, 1904, p. 50. 
