CHAP. XX 
THE BIGGAR ERUPTIVE CENTRE 
327 
not nnlike parts of the Warklaw group of the Pcntlavul Hills, those which 
occur above it are partly andesites and partly orthoclase-felsites. The latter 
form, among the hills near Dolphinton, an important group which reaches 
its greatest development in the Black Mount (1689 feet). These rocks 
cover a breadth of more than a mile of ground, and proliably attain a thick- 
ness of not less than 2000 feet. They so closely resemble in their general 
characters the corresponding rocks of the Pentland Hills that a brief de- 
scri])tion of them may suffice. As in that chain of hills, they are so prone 
to decomposition that they are in large part concealed under a covering of 
their own debris and of herbage, though tlicir fragments form abundant 
screes, and numerous projecting knobs of rock suffice to show the main 
features of the lavas and their accompaniments. 
The felsites weather into pale yellow and greyish “ claystones,” but 
where fresher sections can be procured they often show darker tints of lilac 
and purple. They are close-grained, sometimes flinty, generally porphyritic 
with scattered highly-kaolinized white felspars, but without quartz, often 
presenting beautiful flow-structure, and not infrequently showing a brecciated 
appearance, which in the usual weathered blocks is hardly to be distinguished 
from the breccia of interstratified tuffs. 
A locality where some of these features may be satisfactorily examined 
is a dry ravine in the farm of Bank, on the south-east side of the Black Mount. 
Here the felsite possesses such a perfectly developed flow-structui-e as to 
split into slabs which, dipping S.E. at about 25“, might deceive the observer 
into tbe belief that it is a sedimentary rock. A fresh fracture shows the 
laniinm of flow, many of which are as thin as sheets of paper, to be lilac in 
colour, some of the more decomposed layers assuming tints of grey. The 
felspars and micas are arranged with their long axes parallel to the lines 
of flow. The rock is not vesicular, but it breaks up here and there into the 
brecciated condition just referred to. Below the sheet which displays the 
most perfect flow-structure, what is probably a true volcanic breccia makes 
its appearance. It consists of angular fragments of a similar lilac felsite, of 
all sizes up to pieces two or three inches in length, cemented in a matrix of 
the same material stained reddish-brown. In this breccia the stones show- 
little or no flow-structure. 
Above the group of felsites and felsitic breccias, grey andesites make 
their appearance, like some of those in the Pentland Hills. They are some- 
times extraordinarily vesicular, the vesicles in the body of the rock being 
filled with calcite, agate, etc. Such lavas must have been originally sheets 
of rough slag. The elongated steam-vesicles have been partly filled up with 
micaceous sand and fine red mud that were washed into crannies of the lava 
in direct communication with the overflying water. It is evident that iir the 
northern part of the Biggar centre the succession of volcanic events followed 
closely the order observable in the l‘entland Hills, but on a feebler scale. 
We may sujrpose that the lower diabases and andesites are the equivalents of 
those of Warklaw and Allermuir, that the felsites and breccias were con- 
temporaneous with those of Capelaw, Caerketton and Castlelaw, and that the 
