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568 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boor IV. — 
supervenes, containing rounded fragments of tuff, perhaps lapilli inter- 
mittently ejected from the neighbouring vent, and passing up into a — 
layer of tuff (6), which marks how the volcanic activity gradually 
increased again. It is evident that, but for the proximity of an active 
volcanic vent, there would have been a continuous deposit of black shale, 
the conditions of sedimentation having remained unchanged. In the ~ 
next stratum of shale (7), thin seams and nodules of clay-ironstone 
accumulated round decomposing organic remains on the muddy bottom. 
A brief volcanic explosion is marked by the thin tuff-bed (8), after 
which the old conditions of deposit continued, the bottom of the water 
(as the shale (9) shows) being crowded with ostracod crustaceans, while 
fishes, whose coprolites have been left in the mud, haunted the locality. 
At last, however, a much more powerful and prolonged volcanic explosion 
took place. A coarse agglomeratic tuff (10), with blocks sometimes 
nearly a foot in diameter, was then thrown out and overspread the 
lagoon. 
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Fig. 296.—SEcTION IN WARDLAW QUARRY, LINLITHGOWSHIRE. 
The second example (Fig. 296) brings before the mind a volcanic 
episode of another kind, in the history of the same region. At the 
bottom of the section a pale amygdaloidal, somewhat altered basalt-rock 
(A) marks the upper surface of one of the submarine lavas of the Car- _ 
boniferous Limestone period. Directly over it comes a bed of limestone 
(B) 15 feet thick, the lower layers of which are made up of a dense 
growth of the thin-stemmed coral, Lithostrotion irregulare, which over- 
spread the hardened lava. The next stratum is a band of dark shale (C), 
about 2 feet thick, followed by about the same thickness of an impure 
limestone with shale seams. The conditions for coral growth were 
evidently not favourable; for the deposit of this argillaceous limestone 
was arrested by the precipitation of a dark mud, now to be seen in the 
form of 3 or 4 inches of a black pyritous shale (E), and next by the 
inroad of a large quantity of a dark sandy mud, and drift vegetation, 
