CHAP. XII 
ARENIG ERUPTIONS—SCOTLAND 
I9S 
one of the purplish dialja.se-porphyrites, which occurs in mass on the shore 
and inland, and closely resembles the rock of Carnethy in the Old Eed 
Sandstone volcanic series of the Pentlaiid Hills. 
Yet another and very distinct type of agglomerate is to he seen on the 
Mains Hill south-east of Ballantrae. It is a coarse rock, enclosing blocks 
up to a yard or more in diameter, of a fine compact purplish porphyrite, 
with large crystals of plagioclase and smaller ones of augite. In some 
places immense numbers of the small lapilli in the matrix consist of an 
extremely fine vesicular pumice. Small perfect and larger broken crystals ^ 
of augite are likewise abundant in some of the greenish, more basic parts of 
the mass. These greenish serpentinous parts and the numerous augite 
crystals point to the explosion of some tolerably basic pyroxenic lava. A 
similar dark green, almost black, rock, with augite crystals, which some- 
times measure a (juarter of an inch in diameter, occurs near Sanquhar in 
Nithsdale. It presents a close resemblance to the agglomerate of Khobell 
Fawr, already alluded to. So far as these Scottish agglomerates have yet 
been microscopically examined, they have been found to be composed of 
crystals, crystal-fragments, and lapilli derived partly from lavas similar to 
those above described, and partly from felsitic and other rocks which have 
not yet been observed here in the form of lavas. 
The finer tuffs show likewise a considerable range of composition. 
According to Mr. Peach’s observations along the south-eastern parts of the 
volcanic area, the ejected materials have consisted largely of fine dust 
(probably in great measure felsitic), which towards the north-east is gradu- 
ally interleaved with ordinary sediment till the ashy character disappears. 
As I have already remarked, there is reason to believe that the overlying 
greywackes and shales derived part of their material either directly from 
volcanic explosions or from the attrition of banks of lavas and tuffs exposed 
to denudation. 
But besides the interstratified lavas and fragmental rocks there occur 
numerous intrusive masses which are so intimately associated with the 
volcanic series that they may with little hesitation be regarded as forming 
part of it. They consist of various gabbros and serpentines, which are 
especially developed where the volcanic series comes out in greatest force in 
the south-west of Ayrshire. They also include more acid intrusions which, 
as in the case of the rock of Byne Hill, near Girvan, even assume the 
characters of granite. 
The dying out of the volcanic material towards the north-east probably 
indicates that the vents of the period lay rather in the central or south- 
western parts of the district. Unfortunately, the limited extent of the 
exposures of the rocks makes it a hopeless task to search for traces of these 
vents over by far the largest part of the area. There are two localities, 
however, where the search may he made with better pro.spect of success. 
One of these is a tract to the north of Sanqidiar in Mthsdale, which 
still requires to be studied in detail with reference to the sequence 
and structure of its volcanic rocks. The other area is that south-western 
