328 
BOOK V 
VOLCANOES OF THE LOWER OLD RED SANDSTONE 
last andesites made their appearance together with- those which form the 
highest lavas of the Pentland chain. 
A section across the southern end ot the lliggar volcanic belt shows less 
diversity of structure (Fig. 92). The lavas (3) are there found to flatten 
out and to spread unconformably over tlie older part of the Lower Old 
Led Sandstone (2), which, as already stated, passes down into the Upper 
Silurian sliales. A few intercalations of conglomerate, mainly made up of 
volcanic detritus, are here and there to be detected among these lavas. But 
the chocolate sandstones and conglomerates that lie unconformably below 
IiG. 92. Section across the sontheru part of the Biggar volcanic group from Covington to Ciilter. 
1. Tjower Silurian strata ; 2. Lower Old Red Sandstone (pre-volcaiiie gronri) : 3. Andesite lavas witli intercalated 
sandstones and conglomerates ; 4. Pelsite nock. /, The boundary-fault on northern edge of Sonthern Uplands. 
them contain iio such detritus, for they belong to the pre-volcanie part of 
the history of Lake Caledonia, and were here locally upraised, perhaps as 
an accompaniment of the terrestrial disturbances that preceded or attended 
the first outburst of volcanic energy. Followed south-westwards, the strati- 
graphieal break in the Lower Old Eed Sandstone disappears, and, as will be 
shown in the account of the Duneaton centre, a continuous succession can 
there be traced from the Upper Silurian shales up into the volcanic series. 
An interesting feature in this district is the felsitic boss of Quothquan 
already alluded to (p. 288) as rising up through the andesites, and possibly 
marking one of the vents of the district. It is one of a number of felsitic 
intrusions in this neighbourhood, of which the most important is Tinto. 
A third section taken across Tinto, from Thankerton Moor on the north 
side to Lamington on the south, will serve further to illustrate the great 
Fig. 93. — Section from Thankerton Moor across Tinto to Lamington. 
In. Lower Siluriiiii; 1 . Upper Silurian strata; 3. Lower Old Red Sandstone with two marked bands of con- 
) : *■ Andesite lavas with sn'mistones, 
p" Im ? V-mH I'l.y on JJo. 8 ; .0, Felsite siU ot Tinto wiUi the smaller sill of the 
tap Ciaig ( 0 ). /, Fault boumling the Silurian uplands on the north. A small natch of the uiicoiiforiiiable 
Lower Old Red conglomerate is seen on the south side of the fault. uiicoiiioiiiiacile 
unconforinability in the Lower Old Led Sandstone of this district, and to show 
the relation of the largest felsitic intrusion to the surrounding rocks (Fig. 93). 
Tlie conglomerates and sandstones that appear on the south slopes of Tinto 
