326 
VOLCANOES OF THE LOWER OLD RED SANDSTONE 
KOOK V 
referred to. But after tliat point, wlieii they cross the Lyne Water, they 
begin to be more and more interstratified with thin sheets of andesite. These 
lavas, the beginning of the Biggar series, soon number nine or ten distinct 
bands, and so quickly do they usurp the place of the sedimentary materials 
that in a distance of not more than twelve miles they form, where traversed 
by the river Clyde, the whole breadth of the Adsil)le tract of Old Bed Sand- 
stone, to the exclusion of the conglomerates. 
Unfortunately, soon after the lavas make their appearance at the north 
end they are in great measure overlapped unconformably by the red sand- 
stones at the base of the Carboniferous system, but where the Medwiii Water 
has cut through this covering, they can be seen here and there underneath 
on their southerly course. 
A section through the northern end of the Biggar series, where the 
successive lavas are dying out northwards among the conglomerates, shows 
the structure given in Fig. 91. The sedimentary strata consist largely of 
Fig. 91. — Section across the northern end of the Biggar volcanic group, from Fadden Hill to 
beyond Mcndick Hill. 
1. Conglomerate.s an<l sanilstones ; 3. Lavas, the lowest being an olivine-diabase or basalt, the main mass 
being andesites ; 3. Felsites and tuffs ; 4. Upper Old Red Sandstone. /, Fault. 
debris of andesite, and the lavas include dark red or purple andesites and 
also pale felsites, lioth having the same characters as those of the Pentland 
Hills. 
In one important respect the volcanic series in the northern part of the 
Biggar area differs from that of the Pentland Hills, for whereas the upper- 
most parts of the latter are concealed by faults which liring down the Car- 
lioniferous strata against the base of tlie hills, the lavas at the north end of 
the Biggar district pass conformably under a thick group of Lower Old Red 
conglomerates and sandstones. We thus learn that here the volcanic 
eruptions ceased long before tlie close of the deposition of the Lower Old 
Peel Sandstone. The overlying sedimentary series is disposed in a long 
synclinal trough, corresponding in direction with the general north-easterly 
strike of the volcanic rocks which reajipear from under the sandstones and 
conglomerates along its soutli-eastern liorder, where they are abruptly trun- 
cated by the fault (/, Pig. 92), which brings them against the Hanks of the 
Silurian Uplands. It is interesting to note that hy this dislocation the lavas 
of the Lower Old Red Sandstone are placed almost in immediate contact 
with those of the Lower >Silurian series, which appear here on the crests of 
numerous anticlinal folds that are obliquely cut off by the fault. 
There is yet another feature of interest in the northern part of the 
Biggar volcanic centre. While the lowest visible lava is an olivine-diabase 
