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PROFESSOR T. j. JEHU AMD DR ROBERT CAMPBELL ON 
grained mosaic of quartz often associated with chlorite. The source of these is 
uncertain, but they resemble closely the material which infills many of the vesicles 
of the palagonite fragments. In one section there was noted a broken crystal of 
zircon. 
The matrix in which the above pebbles are embedded consists of small clastic 
quartzes, colourless micas, felspars (both orthoclase and acid plagioclase), along with 
what must have been originally a fine mud, and which now consists for the most 
part of minute flakes of mica, scales of chlorite, and needles of rutile, all probably 
authigenous in character. Locally there is evidence of rather intense silicification, 
resulting in the formation of crystalline quartz round the margin of the original 
quartz pebbles and as authigenous granules in the matrix. To some extent also 
there is local calcification, later than the silicification. 
The Clashbeg breccias may be taken as typical of all the occurrences, but there 
are points of difference in the character of the minor constituents and in the relative 
abundance of the dominant palagonite, quartz, and sedimentary fragments. The 
breccias north-east of Bofrishlie Farm, for example, yield occasional pieces of 
characteristic spilites with the felspars in good preservation ; they contain, too, 
the dark green tourmaline and the pleochroic zircons which are constant accessories 
of the overlying grits. 
The Grits .- — The grits belonging to this series are for the most part rather fine- 
grained, with the grains less than 1 mm. in mean diameter ; occasionally they are 
somewhat coarser, containing pebbles of quartz and shale up to 5 mm. or more in 
diameter. Pieces of the black shales can often be recognised, and occasionally cherty 
fragments derived from the underlying series ; many, specimens are seen to be rich 
in pink felspar : the dominating quartzes are very often black in colour but white 
and blue opalescent varieties are also common ; not infrequently the grits are dis- 
tinctly micaceous. The prevalent colour of the rocks is green or greyish-green, 
changing to rusty brown tints on weathered surfaces. 
In thin section the larger grains (from '2 mm. to 2 mm. in diameter) are seen to 
consist mainly of quartz, felspar, and micas ; zircon and tourmaline are found in 
almost every slide, and composite rock fragments occur occasionally. The grains 
are sometimes well rounded, sometimes subangular. The quartz is for the most 
part typical granitic quartz, with inclusions of fluid cavities, rutile needles, and less 
often tourmaline, biotite, etc. It is invariably the dominant constituent, but in some 
of the arkose-like grits felspar is nearly as abundant. The felspars include microcline, 
orthoclase, perthite, and plagioclase, the last ranging from albite to andesine, but 
being mainly a variety near oligoclase in composition. The larger grains are nearly 
all of alkali felspar, chiefly microcline ; most of the plagioclase grains are less than 
T mm. in diameter, and belong to the finer sand of the matrix, where they are often 
in excess of the orthoclase and microcline, and in the more highly felspathic types 
form no inconsiderable proportion of the rock. On the whole the felspars are 
