File:The Quarterly journal of the Geological Society of London (12733784715).jpg

J. W. JUDD ON THE SECONDARY ROCKS OF SCOTLAND.
279
less regular form, while they often extend to much greater length
than those of the latter class, unlike which again they are never di-
vided into regular blocks by equidistant, curved, joint-planes. One
of the best examples of the columnar structure among the old lavas
Fig. 8.— Cliff-section, 8. W. of Oban, exhibiting very fine Columnar
Structure in the Porphyrite Lavas of Lorn.
of Lorn is that exhibited in a cliff about two miles south-west of
Oban, where the columns are of great length and beautifully curved
(see woodcut, fig. 8)

but more or less perfect examples of the same
structure are exposed on the face of Beinn Lora and at other points.
Some of the volcanic rocks of Lorn present characters which, at
first sight, appear not a little anomalous and puzzling

but this is a
circumstance which, when we remember the metamorphic processes
to which lavas, in common with all other rocks, are subject, need
scarcely occasion surprise. Certain of the lavas appear after their
emission to have weathered into the characteristic spheroidal and
concentric forms, while others have evidently decomposed into a
" wackose " condition

and in both cases the rocks which result from
the re-induration of such masses, present very peculiar features, and
may, in some instances, be mistaken for consolidated "trap-tuffs."
The separate lava-flows were often of enormous thickness. As a
general rule the great body of each of these streams is made up of a'
compact, often highly porphyritic rock

but towards the upper and
under surfaces of the mass, it usually assumes the amygdaloidal
structure. The amygdaloidal cavities, which have evidently served
as chemical laboratories in which very complex operations have been
carried on, are frequently deprived of their contents by recent wea-
thering operations

and the original structure of the rocks is thus to a
great extent restored. Then is made clearly apparent the originally
highly vesicular character of the upper and under portions of these an-
cient lava streams, the vesicles being often seen to be drawn out in the