504 PROFESSOR GEIKIE ON THE 
a petrographical distinction between the older and younger basalts breaks down 
completely when the rocks of the Basin of the Firth of Forth are brought in | 
evidence. Jn their behaviour in the field, and quite as much in their structure 
under the microscope, they cannot be discriminated from these of Tertiary date. 
The specimen from Strathblane which was described by ZIRKEL as a good 
example of an abundant type of true basalt, is an intrusive boss among the 
Carboniferous rocks, and is almost certainly of lower Carboniferous age. 
TV. SERPENTINE-OLIVINE ROCKS. 
Pikrite. (Plate XI. fig. 6.) 
As an appendix to the normal dolerites and basalts, I insert here an account 
of a very remarkable and beautiful rock of rare occurrence in the Basin of the 
Firth of Forth. It is intimately associated with these rocks, and has evidently 
proceeded from some of the same vents. I know it as yet from only two 
localities—Blackburn, near Bathgate, and the island of Inchcolm. It appears 
to agree most closely with some of the rocks from the Fichtelgebirge, 
described by GUMBEL under the name of “ Pikrite.” 
1. General External Characters and Mode of Occurrence.—The variety from 
Blackburn occurs in that interesting belt of ground which runs southward 
through the Linlithgow and Bathgate Hills into the county of Mid-Lothian. This 
tract contains the records of a prolonged volcanic activity, during which lavas 
and tuffs were thrown out to a united depth of many hundred feet. The earliest 
eruptions began before the commencement of the deposition of the Carboniferous 
limestone series, and continued until the greater part of that series had been 
formed. As I have already stated, the lavas consist of basalts and amygdaloidal 
anamesites, the basalts being often singularly fresh, while the more coarsely 
crystalline rocks, especially where amygdaloidal, are often much decomposed. 
The volcanic ridge descends somewhat abruptly into the low grounds at its 
southern end. This arises apparently from several causes. A large fault witha — 
downthrow to the south strikes eastward from the Bathgate coal-field, and skirts 
the southern base of the hills. The volcanic rocks are thus thrown down on the 
south side. Besides this, as the vents of eruption seem to have lain to the north, 
the accumulated sheets of lava and tuff no doubt thinned away in a southerly 
direction. Moreover, a vast amount of sandy and gravelly drift and of underlying — 
boulder clay has been laid down over the lower grounds. That the volcanic rocks, 
however, continue southward is proved by numerous bores which have been sunk 
through the drifts in search of coal. They rise to the surface near the village of 
Blackburn, and after another interruption from the depth of glacial detritus 
are seen in the channel of the River Almond, whence they may be traced — 
southwards for about two miles further. The sections in that stream, as well . 
