CHAP. XII 
ARE NIG ERUPTTONSSCOTLAND 
199 
of lava six inches or more in diameter. Below the belt of lavas come 
black cherts and shales (e) succeeded below by volcanic breccias and tuffs 
(/) alternating with shales in thin inconstant courses. These coarse detrital 
rocks are thororrghly volcanic in origin, and they contain fragments of the 
black cherts which lie still lower in the series. The whole depth of strata 
represented in this section does not amount to much more than 100 feet. 
While in some parts of the Ayrshire district the coarse breccias that 
accumulated around their parent vents form most of the upper part of 
the volcanic series, in others the lavas are succeeded by fine tuffs 
which are intercalated among the ordinary sediments, and show a 
gradual decline and cessation of volcanic energy. South of Ballan- 
trae, for example, the lavas occupy more tlian two miles of coast, in which 
space they display hardly any intercalations of sedimentary material, though 
they show more or less distinctly that they consist of many separate flows. 
Where they at last end, bands of nodidar and fine tuff make their appear- 
ance, together with bands of ashy shale and the characteristic zone of the 
red radiolarian cherts or flints. Above these, in conformable sequence, 
come bauds of black shale, containing abundant Upper Llandeilo graptolites, 
overlain by greenish or olive-coloured shaly mudstones, which pass upward 
into a thick overlying group of greywackes. 
In this section the alternation of fine pyroclastic with ordinary sedi- 
meut shows that the volcanic eruptions in the southern part of the Ballantrae 
district came to an end by a slowly-lessening series of explosions. The 
ashy material gradually dies out, and does not reappear all through the 
thick group of sandy and muddy sediments which here overlies the volcanic 
series. 
We thus learn from the evidence of the Ayrshire sections that volcanic 
action was in full vigour in the south-west of Scotland during the Arenig 
period, but gradually died out l;>efore the end of the Llandeilo period. Ihe 
rocks in which this volcanic history is chronicled have been very greatly 
disturbed and plicated, so that though from their frequently vertical position 
they might bo thought to attain a vast depth, they very possibly do not 
exceed 500 feet in thickness. 
As the volcanic series is followed north-eastwards it exhibits a gradual 
diminution in extent and variety, but this may be at least partly due to the 
much less depth of it exposed on the crests of the narrow anticlines that 
bring it to the surface. There is evidence in that region that the eruptions 
did not everywhere terminate in the Llandeilo period, but were in some 
districts prolonged into the age of the Bala rocks. Thus in the neighbour- 
hood of Sanquhar volcanic breccias, tuffs and lavas have been found by 
IMessrs. Peach and Horne intercalated in strata apparently belonging to 
the Bala group. Again, in the district of Hartfell, a moderately coarse 
volcanic agglomerate occurs in the heart of the so-called “ barren mud- 
stones” of the Hartfell black-shale grouj), which, from its graptolites, is 
placed on the horizon of the Bala rocks. At Wiukstone, Hamilton Hill, 
and Wrae in Peeblesshire, perlitic felsites and soda-felsites have been 
