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Proceedings of Royal Society of Edinburgh. [sess. 
and augite, and is also abundantly scattered through the brown 
glass. Olivine is present as small crystals of simple form (about 
*15 mm.), but does not make up any large proportion of the rock. 
Augite occurs in small crystals and crystalline groups. The largest 
group, consisting of four or five individuals arranged in a more or 
less radial manner, measures about *5 mm. across. In colour, form, 
and mode of grouping, the augite is somewhat similar to that of 
the limburgites. The plagioclase is a basic variety, probably 
labradorite. The sections are, as a rule, lath-shaped in form ; but 
there is a considerable amount of variation in the dimensions of 
the individuals, and in the proportion of length to breadth. The 
sections of some of the larger individuals indicate a tabular form. 
The felspar is remarkably free from inclusions. It far exceeds the 
other crystalline constituents in size, the individuals often measur- 
ing more than a millimetre in length. 
The brown glass forms a considerable portion of the entire mass. 
The colour is not uniformly distributed, being less marked in the 
vicinity of augite and magnetite. 
The third pebble is a more compact basalt in which olivine, if it 
occurs at all, is only represented by a few very minute grains. 
The rock is composed of acicular microlites of felspar, augite 
(scarce), magnetite, and brown glass. The microscopic section of 
this rock contains one large phenocryst of plagioclase. The 
microlitic felspars are arranged with their longer axes approxi- 
mately parallel to one another, thus giving to the rock a marked 
fluidal structure. 
