CHAP, xvni 
ARRANGEMENT OF BEDDED LA VAS AND TUFFS 
283 
Fio. 65. — Veins and neats of sandstone due to the washing 
of sand into fissures and cavities of an Old Red Sandstone 
lava. Turnberry Point. Ayrshire. 
succeeding lava. If this inference be well founded, and it is confirmed by 
other evidence which will be sitbsequently adduced, it points to the probable 
lapse of considerable intervals 
of time between some of the 
oittflows of lava. 
But perhaps the most 
singular structure displayed 
by these lavas is to be seen 
in the manner in which they 
are traversed by and enclose 
portions of sandstone. Since I 
originally observed this feature 
on the Ayrshire coast, near 
Turnberry Point, many years 
ago,^ 1 have repeatedly met 
with it in the various vol- 
canic districts of the Lower 
Old Eed Sandstone across the 
whole of the Midland Valley 
of Scotland. The first and 
natural inference which a 
cursory examination of it suggests is that the molten rock has caught 
up and carried along pieces of already consolidated sandstone. But a 
little further observation will show that the lines of stratification in the 
sandstone, even in what appear to be detached fragments, are marked by 
a cmneral parallelism, and lie in the same general plane with the surface 
of the bed of lava in which the sandy materifil is enclosed. In a vertical 
section the sandstone is seen to occur sometimes in narrow dykes with 
even, parallel walls, but more usually in irregular twisting and branching 
veins, or even in lumps which, though probably once connected with 
some of these veins, now appear as if entirely detached from them 
(Fiw. 65). Frequently, indeed, the nodular slaggy andesite and the 
sandstone are so mixed up that the observer may hesitate whether 
to describe the mass as a sandstone enclosing balls and blocks of 
lava or as a scoriaceous lava permeated with hardened sand. A 
close connection may be traced between these sandstone -inclosures and 
the beds of sandstone interstratificd between the successive lavas. We 
can follow the sandy material downwards from these intercalated beds into 
the andesites below them. On exposed upper surfaces of the lava, an 
intricate reticulation of sandstone veins may be noticed, in each of which 
the stratification of the material runs across the veins, showing sometimes 
distinct current-bedding, but maintaining a general parallelism with the 
bedding of the volcanic sheets and their fragmentary accompaniments (Fig. 
66). If we could remove the sandstone-veinings and aggregates, we should 
find the upper surfaces of these igneous masses to present a singularly fissured 
1 See .Takes' Manval of Geology, 3rd edit. (1872), Fig. Ill, R. 276. 
