J. W. JTJDD ON THE SECONDARY ROCKS OF SCOTLAND. 285
a lava current. The dark-blue highly cleaved slate is changed to a
soft grey rock, divided into angular fragments by numerous joints,
while the brilliant crystals of iron-pyrites with which the slate is
studded are converted into a black amorphous substance still filling
the original cavities (see woodcut, fig. 9, p. 281).
Many examples of the contact of a lava stream with a bed of sand-
stone are exhibited in Lorn and the adjoining islands. The sandstone,
while still exhibiting more or less clearly, under the lens, its original
granular structure, has for some depth from its surface been con-
verted into a substance of intense hardness, and the mixture of fel-
spathie and siliceous materials of which it is composed has evidently
undergone incipient fusion.
"Where the lava has flowed over a bed of conglomerate, the pebbles
of the latter have been frequently caught up and enveloped in the
mass of vesicular and scoriaceous rock forming the base of the lava
stream, in a manner which has often been described as taking place
in the case of the products of recent volcanoes*.
In a road-cutting in the vicinity of Oban I found a very interesting
illustration of the phenomena presented at the contact of some of the
lava streams with the beds below. The section was as follows

—
(a) Compact dark-coloured felstone lava, which has apparently undergone con-
siderable alteration

here and there a hollow vesicle occurs in the mass. A
thickness of 15 or 20 feet of this rock is seen in the section.
(b) About nine inches or a foot from its base this mass of lava becomes highly
vesicular or scoriaceous, the cavities being strikingly flattened and drawn
out. Many burnt-looking fragments (lapilli) are caught up in this part of
the lava stream.
(c) For a depth of about 2 feet below we have a confused mass of vesicular lava,
rounded blocks of lava, pebbles of quartz, &c, all imbedded in a matrix of
highly scoriaceous rock. The lower part of this mass passes in places
into a sandy conglomerate

the ordinary coarse sand consisting of lava de-
tritus constituting the matrix.
(d) Conglomerates, composed of pebbles of trap and of the Lower Silurian rocks,
with some unmistakable volcanic bombs, and with a matrix of very coarse
sand, 2 feet thick. In this bed are lenticular patches of a very fine-grained
sedimentary material, probably stratified volcanic dust.
(e) Beds of well-stratified and finely laminated red sandstone, very similar in
character to the typical rocks of the Old .Red Sandstone.
The stream of lava of which (a) and (b) form the lower portion
appears to be 40 or 50 feet in thickness, to be highly vesicular in its
upper part, and to be covered directly by another lava stream. The
appearance presented by (6) and (o) is exactly such as would be
produced by a lava stream with a cindery crust which it rolls over
as it flows along, in the manner so graphically described by Mr.
Scrope, and passing over a mass of shallow -water deposits, the mate-
rials of which it entangles in its course.
The sandstones and conglomerates, which in places attain to a
thickness of several hundreds of feet, appear to be wholly confined to
the lower part of the volcanic series of Lorn. In its upper part

In the island of Lipari there occur beautiful instances of lava streams of
glassy character having entangled in their mass many fragments of the older
rocks, that have evidently strewed the surfaces over which they