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280 J. W. JUDD ON THE SECONDARY ROCKS OF SCOTLAND.
direction of the flow, while the actual surfaces of the stream exhibit
the most strikingly scoriaceous aspect. The same weathering pro-
cess sometimes develops in these old lavas other original structures,
which had become wholly obliterated by infiltration, crystallization,
and other processes taking place in the mass of the rock

and these
structures would have remained altogether unsuspected but for the
action of this cause. Thus some highly crystalline and porphyritic
rocks, when weathered, resume their earthy or compact texture

and
in certain cases structures like the ephaerulites of pearlstone, which
had become wholly obscured in the mass, are again revealed. Simi-
larly many rocks of very solid appearance are seen, when the infil-
trated materials are removed by weathering, to have been originally
aggregates of ashy, pumiceous, and scoriaceous fragments, among
which " volcanic bombs " or their fragments may not unfrequently
be detected*.
3. Relations of the Volcanic Roclcs of Lorn. — In considering this
question it is important at the outset to notice two striking facts with
regard to the positions of the lavas and associated beds of Lorn. In
the first place they always rest directly, but unconformably, upon
the Lower Silurian gneissose and schistose rocks, and never exhibit
any of the fossiliferous Secondary strata at their base

in this respect
they present a striking difference in their relations from the Terti-
ary lavas of the adjoining districts. And, secondly, they are, like
the older rocks upon which they repose, penetrated in every direc-
tion by numerous dykes of dolerite and basalt

these are precisely
similar to the intrusive masses associated with the Tertiary volcanic
rocks — of which series, indeed, we can scarcely hesitate to regard
them as forming a part.
These later dykes of dolerite and basalt, which are sometimes of
great width, and often present a prismatic or columnar structure, con-
stitute a most interesting and striking feature in the district. In
consequence of their usually greater relative hardness, they frequently
stand up like gigantic walls amidst the rocks of slate and felstone
which once enclosed them but have now been weathered away from
their sides.
The remarkable series of volcanic rocks of Lorn is probably several
thousand feet in thickness — though, its upper portions having been
everywhere removed by denudation, its original limits in this respect
are quite unknown to us. Its relations to the slate rocks and certain
conglomerates, sandstones, and breccias appear, at first sight, to be

That the geologist never meets with glassy lavas (obsidians, pitchstones &c.)
among the older rocks will not occasion any surprise when we reflect upon the
facility with which artificial glasses undergo devitrification. That glassy lavas
were formed even in palaeozoic times appears to be indicated by the fact recorded
in the text that weathering sometimes reveals the characteristic sphcerulitic struc-
ture in some of the lavas of Lorn. Many of the Newer Palaeozoic " porphyrites "
of Scotland are quite undistinguishable in appearance from some trachytes, such
as those of Hungary. The white granular siliceous rocks into which the former
are sometimes found altered, appear to be equally undistinguishable from the
products of the decomposition of the latter (occasioned by the passage of acid
vapours through them), and constituting the so-called