CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 479 
intrusive sheets, and differs from that of interbedded masses, that we may 
regard it as having been once covered by rocks under which it was injected. 
Since, however, isolated portions of columnar basalt occur in this and some of 
the neighbouring masses, we have probably to do with phenomena both of a 
hypogene and superficial character. These basalts may have flowed out at the 
surface from one or more of the surrounding vents, though the main sheets of 
dolerite were intrusive. This association of intrusive and interbedded masses 
round the same volcanic centre would be precisely analogous to that at Arthur 
Seat. The microscope furnishes a satisfactory means of discriminating between 
many of the intrusive and interbedded igneous rocks, as will be explained in the 
sequel. 
Age of the Intrusive Sheets.—As the great epoch of volcanic activity in the 
Basin of the Firth of Forth extended from an early part of the Calciferous 
Sandstone period to near the close of the Carboniferous Limestone, it is probable 
that a large number of the intrusive sheets belong to some portion of that 
protracted series of volcanic eruptions. But some of them are certainly later 
than the latest known member of the Scottish Carboniferous system. These 
acquire a fresh interest and importance when viewed in connection with the 
evidence already given in this memoir regarding post-Carboniferous, or at least 
very late Carboniferous, volcanic action in Fife. For it is manifest that the 
phenomena were not confined to one small area. These late intrusive sheets 
cover a large area in Linlithgowshire and Lanarkshire. They reappear in 
Ayrshire, where the Coal-measures are likewise pierced with volcanic vents, and 
overspread with sheets of lava, tuff, and sandstone of Permian age. It is quite 
possible also, as I have already remarked, that some of the volcanic necks 
between the east of Fife and the heart of Ayrshire may belong to this late 
period. The later agglomerate of Arthur Seat has so exactly the character of 
some of the Permian necks of Ayrshire, that I suggested some years ago the 
probability of its being of the same age. More recently I have observed on 
Largo Law and others of the latest necks in the east of Fife, agglomerate 
precisely of the Arthur Seat type, which is coarser and redder, with more of 
the debris of the surrounding rocks, and less of the dirty-green diabasic matrix 
than the ordinary agglomerate of the older Carboniferous necks. 
I have referred to the sheets of dolerite and basalt which in the east of Fife 
spread over considerable areas of the surface, and stand in evident relationship 
with the volcanic necks around them. In no material respect do they differ 
from the sheets which invade and overspread the upper Coal-measures in the 
great coal-field. In both cases they overlie the Carboniferous strata, and are not 
themselves covered by any later formation except glacial drift and other post- 
Tertiary deposits. All therefore that can be affirmed regarding them is that 
they are later than the youngest part of the Scottish Carboniferous system 
