File:The Quarterly journal of the Geological Society of London (12733950803).jpg

266 J. W. JUDD ON THE SECONDARY HOCKS OF SCOTLAND.
sequence of their greater hardness, an almost entire belt of the
altered and indurated basalts is found surrounding the crystalline
mass on all sides. Prom the central mass a number of dykes and
veins can be traced intersecting the surrounding older lavas.
That, subsequently to the eruption of the great sheets of basaltic
lavas, a mass of basic igneous rock was upheaved in their midst at
Sarsta Beinn is evident. And that this eruptive mass was surmounted
by a volcanic cone from which probably lava streams flowed, no one
who has studied the example of Beinn Shiant will, I think, be disposed
to doubt. Indeed fragments of these lava-streams may still remain,
though now forming undistinguishable portions of the great basaltic
plateau. When we remember the observation of Mr. Darwin, that
both in the Cape-Yerde Islands and the Galapagos archipelago he
found it impossible to trace the boundaries of recently erupted lava
streams, " except by the size of the bushes growing upon them, or
by the comparative glossiness of their surfaces, characters which a
short lapse of time would be sufficient to obscure," it will not be sur-
prising that we are not, as a rule, able to trace the separate lava
streams of the Hebrides. The fact that in the case of the lavas of
Beinn Shiant we are able so to do, is due solely to the fact that in
them a great capacity for resisting degrading influences is combined
with remarkably distinctive penological characters.
I have described in some detail these two examples of Beinn
Shiant and Sarsta Beinn on account of their great size and typical
character

but there is the clearest evidence that at a very great num-
ber of points the basalts of the great plateau were broken through
by similar eruptive masses, sometimes, as in the case of Sarsta
Beinn, composed of basic rocks, at others, as in Beinn Shiant, of more
felspathic materials, and in others, again, of both these varieties. Few
of these later eruptions appear to have been on the same grand scale
as those which we have selected as types

and many of the intruded
rock masses, which now alone remain to indicate their points of
origin, are of quite insignificant proportions. Thus in the little
island of Eigg two such masses were detected by Professor Geikie,
and I have found a third of similar character

iu the other islands
also such masses are found scattered in all directions. Until, how-
ever, the whole district has been mapped in the most careful detail,
it will be impossible to give a distinct view of the distribution
of these latest points of eruption in the district, and to show their
exact relations to the great volcanoes. Even after such survey,
indeed, many of the smaller intrusive masses may escape the obser-
vation of the geologist through being concealed by peat-mosses or the
vegetable covering.
Having pointed out that, even in the great mass of Beinn Shiant,
the preservation of the ruins of the volcanic cone are due to very re-
markable and exceptional circumstances, it will be almost unneces-
sary to add that the small cones which doubtless once surmounted
most, if not all, of these eruptive masses, have in almost every instance
been wholly swept away by denudation, and that the lavas which
flowed from them have also disappeared or are now