PETROLOGICAL ABSTRACTS AND REVIEWS 
EpitED By ALBERT JOHANNSEN 
CRAIG, WRIGHT, BAILEY, CLOUGH, AND FLeTr. The Geology of 
Colonsay and Oronsay, with part of the Ross of Mull. Mem. 
Geol. Survey Scotland, No. 35. Edinburgh, 1911. Pp. viii 
—200;. plates Vis figs. 21> “map E: 
Colonsay and Oronsay are two small islands of the Inner Hebrides, 
lying between Islay and Mull, and are formed chiefly of schistose, meta- 
morphic rocks of sedimentary origin and probably of Lower Torridonian 
age. They include limestones, phyllites, mudstones, banded flags, 
sandstones, feldspathic and epidotic grits, and conglomerates, and have a 
thickness estimated to be at least 5,000 feet. The rocks are much folded 
and show two series of cleavages, the earlier of which is slaty cleavage and 
is separated from the later “‘strain-slip” by a period of igneous activity, 
during which time there were intruded several small masses of syenite 
and diorite and numerous lamprophyric dikes. Subsequently a series 
of vogesites were intruded. No sediments are found intermediate 
between the Lower Torridonian and those of the Glacial period, the 
long interval being represented only by the igneous intrusions. 
The igneous rocks consist of quartz-hornblende syenite, kentallenite, 
and augite diorite. There are two phases of the syenite; a marginal 
phase which is very basic and full of included bowlders, and an interior 
acid phase which is bowlder free. The marginal phase is a dark 
rock consisting of short, stout crystals of hornblende and a little biotite 
in a scanty matrix of feldspar (perthitic orthoclase, with albite in 
some places) usually micrographically intergrown with quartz. A 
peculiar feature of this border facies is the fact that it is crowded with 
bowlders of quartz and quartzite in all stages of assimilation, the unal- 
tered portions being surrounded by halos of feldspathic material, usually 
potash feldspar and quartz with a little albite, formed by the combina- 
tion of the dissolved silica with the basic magma. The “feldspathic 
ghosts”’ often retain the original shape of the bowlders and indicate the 
tranquillity of the process of replacement and the viscosity of the magma, 
the later being indicated also by the uniform distribution of the lighter. 
quartz bowlders in the heavier magma. The authors say the rock can 
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