CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 505 
as in the Briech Water, instructively show how rapidly the volcanic sheets 
diminish southwards. Of the vast mass of basalt and tuff intercalated 
between the lower limestones, most of that to the east of Bathgate has dis- 
appeared. It is the upper or later portion of the volcanic series, lying above 
the Main Limestone, which is prolonged southwards into Mid-Lothian, its 
position being indicated by the thin limestones between which it is intercalated. 
At Blackburn one of the old lavas of a very peculiar kind has been quarried 
for many years as a material termed “lakestone,” employed for the construction 
of the soles of ovens, owing to its capability of withstanding the effects of 
considerable heat. The rock is seen only at the quarry itself. It is there 
_found to present a beautifully ice-worn upper surface, lying under a mass of 
stiff dark boulder-clay. In the channel of the Almond Water, immediately 
below the rock, some shaly sandstones and shales occur, with a thin seam of 
crinoidal limestone lying upon a few inches of coal. These strata dip towards 
the north-west at 25°, and the volcanic sheet has a similar inclination. It is 
evidently a bed intercalated in the limestone series. Its precise stratigraphical 
relations are made clear by a section in the Skolie Burn, less than two miles 
further south. A rock of a similar character, and quarried for the same 
purpose, is there exposed under a group of calcareous shales and thin lime- 
stone. Its upper part is in some places a fine slaggy amygdaloid. The strata 
lying directly upon it are of a peculiar green felspathic sandstone or shale, con- 
taining detached fragments of the amygdaloid, and likewise Lingula and other 
mollusca. It is clear that the rock is not an intrusive sheet, but is a true lava- 
bed, which was erupted and solidified at the surface during the accumulation of 
the older part of the Carboniferous Limestone series of West Lothian. 
I am thus particular regarding the geological relations of this rock, because 
it stands at present as a nearly unique example of a very interesting variety 
of these Carboniferous lavas, and likewise offers a striking petrographical 
difference between the upper and under part of the same bed, such as I have 
not been able to discover anywhere else. 
The rock is best displayed at the Blackburn Quarry. Looked at from a 
short distance, it appears to be one of the rudely jointed, somewhat decomposed 
brown or dirty-green doleritic or diabasic rocks of the district, with a tendency 
to weather out into spheroids. Examined more closely, the upper portion is 
seen to bear out this first impression, though evidently to present more decided 
traces of serpentisation than is usually to be observed. A specimen taken 
from the lower body of the stone would be termed a highly serpentinous 
diabase, nearly approaching a serpentine in outward aspect. Veins of serpen- 
tine and chrysotile, sometimes six inches thick and often streaked with calcite, 
run in vertical divisions through the whole rock. There is no line of demarca- 
tion to be drawn between the higher and lower parts of the rock. They cannot 
VOL. XXIX. PART I. 60 
