CHAP. XX 
THE BIGGAR ERUPTIVE CENTRE 
325 
from the southern edge of the vent. The smallest of them measures about 
500 feet in diameter; the largest is oblong in shape, its shorter diameter 
being about 500 feet, and its longer about 1000 feet. The materials that 
fill Siese lateral vents are coarse agglomerates, traversed by veins and 
irregular intrusions of a fine horny or flinty felsite. 
From the acid character of most of the rocks that now fill the wide vent 
of the Braid Hills it may be inferred that at least the last eruptions from it 
consisted chiefly of acid tuffs and lavas. The upper portion of the volcanic 
series being everywhere concealed, there are no means left to verify this 
inference from an examination of the ejected material. It may be remarked, 
however, that the pale yellow sandstones which lie on the east side of the 
fault and are exposed in the Lyne Water above West Linton are in great 
measure composed of fine felsitic material.^ They certainly belong to a 
higher horizon than the most southerly lavas of the Pentland Hills, and 
if they have not derived their volcanic detritus from the Biggar volcanic 
area, it may be assumed that they obtained it from the vent of the Braid 
Hills. In any case they show that after the lavas of the southern end of 
the Pentland Hills were buried, acid volcanic detritus continued to be 
abundantly distributed over this part of the floor of Lake Caledonia. 
6. The Biggar Ccntrc’- 
,\nother distinct group of volcanoes had its centre about 25 miles 
south-westward from the Braid vent, and on the same line as those of the 
Pentland Hills. In no part of the basin can the isolation of the different 
volcanic clusters be so impressively observed as in the area to the south- 
west of these hills. On the one hand, the lavas and tuffs from the Braid 
vent die out, and on the otlier, as we follow the conglomerates south-west- 
wards, a new volcanic series immediately makes its a]ipearance. 
The space between the last extremity of the .Pentland lavas and the 
beginning of the Biggar series does not exceed some 500 yards. It will be 
remembered that the lower half of the Pentland volcanic series dies out long 
before it reaches the southern end of the hills, and that it is by lavas on the 
horizon of some of the dark andesites of Allermuir Hill that the volcanic 
baud is finally prolonged to its extreme southern limit. Tlie most northerly 
extension of the Biggar lavas lies somewhere on the same general platform. 
But whereas, at the north end of the Pentland chain, the volcanic sheets 
rest on the edges of the Upper Silurian shales, at the south end, several 
hundred feet of coarse conglomerate and sandstone intervene between the 
Silurian shales and the porphyrites. So rapidly does the bulk of these 
sedimentary formations increase that in the course ot twm miles they must 
be MOOO feet in thickness below the most northerly of the Biggar lavas just 
1 Explanation to Sheet 24 of the Geological Survey of Scothmil, pp. 10, 12. 
This area is included in Sheets 23 and 24 of the Geological Survey of Scotland. It was mapped 
and described by myself. (Explanations of Sheets 23 and 24.) Various parts of it have been 
referred to by earlier writers, particularly IVIaclaren, Oeology of ^ ife, etc., p. 176, 
