284 
VOLCANOES OF THE T.OWER OLD RED SANDSTONE 
BOOK V 
<and slaggy appearance, reminding us of the rugged, rent and clinker- 
loaded slopes of a modern viscous lava, like some of those in the Atrio del 
Cavallo on Vesuvius. 
There cannot, there- 
fore, he any doubt 
that the sandstone, 
so irregidarly dis- 
persed throng! 1 these 
lavas, was introduced 
originally as loose 
sand washed in from 
above so as to fill 
the numerous rents 
and cavernous inter- 
spaces of the volcanic 
rock. A more strik- 
ing proof of the sub- 
a(]ueous character of 
tlie eruptions could 
Fi<!. 66.— Gronnil-plan of reticulated cracks in tlie upper surface of an Old conceived. 
Red Sandstone lava tilled in with sandstone. Red Head, Forfarshire. This interesting feat- 
ure ill lavas erupted 
under water is not (tonfined to the volcanic series of the Old Eed Sandstone. 
We shall find that it is hardly less distinct among the basic lavas of the 
Permian series both in Scotland and in Devonshire. 
A remarkiible exception to the general tj'pe of dark basic and inter- 
mediate lavas is furnished by the pale, decomposing felsites of the Pent- 
land and Dolphinton Hills. Tlioso which issue from tlie great eruptive 
centre of the Braid Hills, alternate with the andesites and the diabases, 
gradually diminishing like these in a southward direction and dying out 
in some six or seven miles. Beyond the limits of these lavas, another 
similar thick group was erupted from a separate vent at the northern end 
of the Biggar district near Dolpliinton. The same occurrence has been 
ascertained also in the area of the Ochil chain. Fuller reference will be 
made to these interesting rocks in the descriptions to be afterwards cdven 
of the structure and history of the volcanic areas of the Pentland Hills, the 
Biggar centre and the Ochil Hills. 
It is certainly a notable feature in the volcanisni of Old Bed Sandstone 
time that from the same, or from closely adjoining vents, lavas should he 
alternately poured forth, differing so much from each other, alike in chemical 
composition and petrographical characters, as andesites and diabases oil' the 
one hand, and felsites on the other. Additional examples, from widely 
different geological systems, will be cited in subsequent pages. It 'ivill be 
shown that even in the very latest volcanic period in Britain, that of 
older Tertiary time, highly basic and markedly acid materials were ejected 
from tlie same centres of eruption. 
