File:The Quarterly journal of the Geological Society of London (12733807025).jpg

J. W. JUDD ON THE SECOXDA.EY EOCKS OP SCOTLAND. 293
composed of hornblendic granite, passing by insensible gradations
into ordinary granite on the one band, and into syenite granite on
tbe other. The normal character of this rock is that of an aggregate
of white orthoclase and oligoclase felspar with colourless quartz and
hornblende, the latter being usually in part replaced by varying
proportions of lepidomelan

and the whole mass is rendered beauti-
fully porphyritic by the dissemination through it of fine crystals of
orthoclase felspar of a pink colour. Locally, however, it exhibits
many variations from the typical character. It is traversed, too, in
every direction by veins of various size, composed usually of
granite of finer grain, of euritic felsite, or of crystallized quartz and
felspar. The granitic rocks constitute those great spurs, with sharp
summit-ridges and steeply sloping sides, which divide the deep corries
that form so striking a feature in the Beinn Nevis group.
Let us now ascend the central peak and examine its structure.
Some distance above the well-known lake, which lies in a hollow
upon the shoulders of the mountain, a remarkable change is found
to take place in the character of the granitic mass

it becomes much
finer-grained, and as we still ascend we find the hornblende and
mica gradually disappearing, till in the end the rock becomes a finely
granular felsite of a pale red colour, and often more or less porphy-
ritic in structure.
The highest portion of the mountain, however, is "composed of a
mass of rocks of totally different character. Instead of the pale red
granites, eurites, and felsites, we find dark-blue, grey, greeenish,
and purplish felstones

and associated with these are enormous
masses of volcanic agglomerate, composed for the most part of
angular fragments of all sizes, of felspathic materials, heaped
together in the wildest confusion, and compacted into masses of
great solidity and hardness.
The confusion produced by the ever descending fragments which
cover the upper slopes of the mountain and constitute such im-
pressive evidence of the potency of atmospheric agencies in the
work of denudation, is inimical to a complete study of the relations
of the rocks forming the summit of the mountain

but an atten-
tive examination both of these slopes and of the more accessible of
the precipices surrounding the corries shows that the felstones form
great sheets, sometimes exhibiting a rudely columnar structure,
and that between them lie the enormous masses of volcanic agglo-
merate, the whole being traversed by innumerable felstone dykes.
From the same cause, namely the abundance of debris on the
higher slopes of the mountain, the relations of the underlying
felsites and granite to the great cap composed of felstones and
agglomerates is not at first sight very apparent. Careful study,
however, shows that the former rocks send off veins into the latter

and the abundance of the fragments of the granitic rocks, above the
level at which the lavas and agglomerates commence, shows that
these veins are by no means few in number.
With regard both to the felstone lavas and the associated agglo-
merates, the changes produced by the chemical action that has taken
Q. J. G. S. No. 119.