CHAP. XII 
ARENIG ERUPTJONS^SCOTLAND 
197 
flinty shales and thin limestones, which sometimes fill in the hollows 
between the pillow-likc blocks above referred to. Among the characteristic 
Lower Arenig graptolites of these intercalated layers are Tetragraptus hryon- 
oides, T. fruticosus, T. qumlrihracliiatus, and T. Heaxli together witli Caryocaris 
Wriyktii. Considerable variation is to be seen in the development of the upper 
part of the volcanic series. In some places the lavas ascend almost to the 
top ; in others, thick masses of breccia or agglomerate take their place. These 
fragmentary materials are locally developed round particular centres, which 
probably lie near the sites of active vents whence large quantities of pyro- 
clastic material were discharged. One of the volcanic centres must have 
been situated close to the position of Knockdolian Hill already referred to. 
The exceedingly coarse breccia of that eminence is rudely stratified in 
alternations of coarser and finer material, which was probably to some extent 
assorted under water around the cinder-cone that discharged it. The 
date of the explosions of this hill has been ascertained by Mr. Peach from 
the intercalation of black shales containing Arenig graptolites among the 
breccias. Another vent lay somewhere in the immediate neighbourhood of 
Rnockdolian- k! 
the Mains Hill agglomerate, if not actually on part of the site of that rock. 
Though probably not more than a mile from the Knockdolian volcano, and 
belonging to the same epoch of eruption, this vent, to judge from the 
peculiarities of its ejected material, must have been qnite distinct in its 
source. A third vent lay somewhere in the immediate vicinity of Piennane 
Head, and threw out the extraordinary masses of agglomerate and the sheets 
of lava seen on the coast at that locality. A fourth may be traced by' its 
separate group of fine tuffs on the coast three miles south of Ballantiae. 
A feature of singxdar intere.st in the material erupted from these various 
centres of activity' consists in the evidence that the explosions occurred at 
intervals during the deposition of the Lower Silurian formations, and that 
these formations were successively disrupted by submarine explosions. Mr. 
Peach has found, for example, abundant pieces of the peculiar and easily 
recognized radiolarian elierts imbedded in the volcanic series. That these 
cherts were deposited contemporaneously with the volcanic eruptions is 
proved by their intercalation among the breccias. Let among these very 
breccias lie abundant fragments of chert which must have already solidified 
before disruption. It is thus evident that this siliceous ooze not only 
accumulated but set into solid stone on the sea-floor, between periods of 
