IGNEOUS GEOLOGY OF THE BATHGATE AND LINLITHGOW HILLS. 361 
The Second Volcanic Zone, or The Longmumr and Riccarton Lavas. 
This zone is taken to include two apparently distinct groups of lavas. The 
lower or Longmuir group extends from a point a little to the east of Broomyknowes, 
through the Longmuir and Balditop plantations, to Drumcross. The upper or 
Ricearton group is of greater thickness but of less extent, stretching only from the 
Riccarton Burn to the Rigghead plantation, a little to the south of Tartraven. The 
two groups are separated by a series of sandstones, shales, and thin ashbeds, well seen 
in the streamlets and quarries on the northern slope of the Riccarton Hills. Petro- 
graphically, the Longmuir lavas are throughout fine-grained olivine-basalts, while the 
Riccarton lavas can be separated into a lower zone of coarse-grained olivine-dolerites* 
extending the full length of the group, and an upper zone of olivine-basalts stretching 
from the Riccarton Burn to North Mains. 
This subdivision of the zone is everywhere apparent to the north of the Mains 
Burn, but to the south so little rock is exposed that the lines are to some extent 
hypothetical. ‘The sedimentary intercalation between the two groups of lavas cannot 
be traced by means of actual exposures of sandstones and shales. In consequence, 
more than usual reliance must be placed upon the prolongation to the south of the 
petrographical variations established above. The coarse-grained olivine-dolerites of 
the lower zone of the Riccarton lavas are nowhere found to the south of the Rigghead 
plantation, beyond which the exposures are all of olivine-basalts similar to the Long- 
muir lavas to the north. The sedimentary band, apparently reduced in thickness, is 
therefore drawn, as nearly as possible, between the basalts and the dolerites. Marginal 
sedimentary intercalations are probably numerous throughout this zone. Two such 
are shown on the map, one of shales and sandstones found in drains on the farms 
of Drumcross and Quarter, the other of sandstones, shales, fine-grained green ash, and 
agglomerate, exposed in the Brox Burn at the Balditop plantation. The sections 
exposed during the construction of the Bangour reservoir indicated considerable dis- 
turbance of the strata at that pomt. Another thin bed of sandstone, interstratified 
with the lavas, may be seen immediately to the east of the neck in the Riccarton Hills. 
This second volcanic zone cannot be definitely traced to the south of the Galabraes 
fault, although the section at Starlaw indicates the occurrence of scattered lava-flows. 
North of Broomyknowes also the lavas rapidly run out, and their place is taken by 
interstratifications of sandstone, shales, and thin ashbeds, with at least one thin band 
of shelly limestone exposed in the burn below Riccarton Mull. 
Between the second and third volcanic zones there comes another sedimentary 
intercalation, represented by shales and thin ashbeds at Craigs, limestone and shale 
at Tartraven, shales and ash in the Riccarton Burn west of Beecraigs, sandstone, shale, 
and limestone at Whitebaulks, sandstone at Hillhouse, sandstone and limestone at 
* The terms “basalt” and “dolerite” are used throughout to denote macroscopic distinctions, “basalt ” implying 
a fine-grained, compact, and usually porphyritic rock, “dolerite” a coarse-grained rock, not evidently porphyritic. 
