82 PETROLOGICAL ABSTRACTS AND REVIEWS 
or plum-coloured; it is pleochroic . . . . and the extinction angle about 44°. 
The pyroxene in fact belongs to the variety usually described as titaniferous 
and much resembles that which occurs in many basic nepheline-rocks and 
teschenites. Chemical analysis proves that the crinanites are rich in titan- 
LUTTE The felspar has albite (rarely Carlsbad or pericline) twinning 
and belongs mostly to labradorite, though the outer zones are more rich in 
soda and may consist of oligoclase or albite. The iron oxides form irregular 
plates often fringed with small scales of dark brown biotite. 
Most of these rocks have very perfect ophitic structure, and the augite 
occurs as small angular patches between the lath-shaped felspars or enclosing 
WS 6 aca a In a few specimens there are large corroded felspar phenocrysts 
consisting mainly of bytownite. Analcite and radiating clusters of zeolites 
fill up spaces bwteeen the felspars or occupy small rounded steam cavities. 
Perfectly transparent analcite is not uncommon, but often this mineral is 
turbid and granular with weak double refraction. The radiate zeolite appears 
to be mostly natrolite. Evidently these have been the last minerals to crystal- 
lise, and as the rocks are often very fresh, it is difficult to believe that they have 
originated from the decomposition of the felspar. They are more properly 
a pneumatolytic infilling of interstitial spaces during a period immediately 
following the crystallisation of the pyrogenetic minerals. Carbonates and 
chlorite are often associated with them, and veins of analcite and zeolites, 
easily distinguished by their low refractive indices, often ramify through the 
substance of the felspar. 
In their composition and in the properties of their minerals these crinanites 
bear much resemblance to the teschenites . . . . but the teschenites are much 
coarser-grained, less frequently porphyritic and contain much more alkali 
felspar. The teschenites occur as large sills or laccolites, the crinanites as 
narrow vertical dykes which often can be followed for long distances in nearly 
straight lines. The crinanites in Colonsay show transitions to the camptonites, 
and are associated with monchiquites, some of which contain nepheline. 
Monchiquite occurs in dikes and is composed of altered olivine, 
biotite, hornblende, and augite in a groundmass of analcite and carbon- 
ates. Anephelite ouachitite also occurs. The “felsite” dikes are described 
as “for the most part too decomposed for petrological examination.”’ 
The authors further describe the tectonics of the islands, their 
glaciation, and their economic resources. 
In Part II the geology of the south part of the Ross of Mull is briefly 
described. The rocks here consist of metamorphosed sediments, intru- 
sive granite and diorite, and dikes of vogesite, porphyrite, monchiquite, 
camptonite, “dolerite,” and granophyre. 
ALBERT JOHANNSEN 
