194 
THE SILURIAN VOLCANOES 
BOOK IV 
rig. 51 represents in ground-plan a surface about twelve feet square on 
the shore immediately to the south of the mouth of the Eiver Stinchar. 
In the heart of the spheroids enclosed fragments of other lavas are some- 
times observable. 
This singular structure has already (p. 184) been referred to as 
strikingly displayed in a rock at the top of Cader Idris. It is found in 
dark basic lavas probably of Arenig age, which will be afterwards referred 
to as occuri'ing along the southern Hanks of the Scottish Highlands and 
also in the north of Ireland. It has been observed by Mr. Teall among the 
rocks of the Lizard, and has been described as occurring in Saxony and 
California.^ In these different localities it is associated with jaspers and 
cherts, some of which contain abundant Kadiolaria. The same structure has 
been found among the variolitic diabases of Mont Genevre,^ and likewise in 
some modern lavas, as in that of Acicastello already referred to {ante, p. 2G). 
The volcanic agglomerates and breccias, in the south-west of Ayrshire, 
Fig. 52. — View of Kiiockdoliau Hill from the east. 
attain a great development in several centres probably at or near the 
original volcanic vents. They present several distinct petrographical types. 
The remarkable neck-like hill of Knockdolian in the Stinchar Valley is made 
of a coarse breccia composed mainly of angular pieces of dull greyish-green 
fine-grained diabase. The breccias and agglomerates of Eennane Head in 
some parts consist largely of broken- up shales, flinty mudstone, black 
radiolaiian flint oi' chert, and abundant fragments of andesites and felsites. 
In other parts the volcanic material predominates, including angular and 
subangular fragments of various somewhat basic lavas, lumps of vesicular slag 
and pieces of pumice. Here and there much calcite is diffused through 
the matrix in strings, veins and patches, which enclose the lapilli. The 
agglomerate north of Lenclalfoot possesses a greenish, somewhat serpentinous 
matrix, through which immense numbers of tabular felspar crystals are 
scattered. Similar crystals also occur abundantly in embedded blocks of 
' Mr. J. J. H. Teall, Roy. OeoL Soc. Cornwall, 1894, p. 3. Mr. L. Ransome, Bnll. Depart, 
Geol. University of California, vol. i. p. 106. 
^ Messrs. Cole and Gregory, Quart. Journ. Geol. Soc. vol. xlvi. (1890), p. 311. 
