478 PROFESSOR GEIKIE ON THE 
Kinghorn on the other. In Linlithgowshire and Edinburghshire, as well as in 
the south of Fife, they traverse the Calciferous Sandstone groups. If the 
horizon of the sheets furnished any reliable clue to their age, it might be 
inferred that they were intruded during the earlier portion of the Carboniferous 
period. But on closer examination it will be observed, that the same intrusive 
mass sometimes extends from the lower into the upper parts of the Carboniferous 
groups. ‘Thus, in the west of Linlithgowshire, a large protrusion which lies 
upon the upper limestones, crosses most of the Millstone Grit, and reaches up 
almost as high as the Coal-measures. Again, in Fife, to the east of Loch Leven, 
a spur of the great Lomond sheet crosses the Carboniferous limestone, dis- 
regards a large fault, and advances southward into the coal-field of Kinglassie. 
In Stirlingshire and Lanarkshire, numerous large dolerite sheets have invaded 
the Millstone Grit and Coal-measures, including even the upper red sandstones, 
which form the top of the Carboniferous system in this region. 
Relation of the Intrusive Sheets to the Volcanic Centres.—Some light might 
be expected to be thrown on the age of the intrusive sheets by the manner in 
which they are related to the various centres of volcanic activity, but a satis- 
factory connection can hardly be established between them. Where the 
intrusive sheets reach their greatest development there are few, sometimes no 
trace, of volcanic pipes, with their associated beds of tuff and streams of lava. 
On the other hand, in those tracts where volcanic orifices must have been active 
for long periods, intrusive sheets, when they occur at all, are commonly small 
and unimportant. In the case of a hill like Arthur Seat, where the lowest 
igneous rocks are intrusive sheets, and where the higher and larger mass 
consists of lavas and tuffs, erupted at the surface, we might speculate on the 
probability that the lower sheets of molten rock had been injected between the — 
planes of the strata, either during some of the preliminary hypogene efforts, 
before direct communication had been established with the surface, or subse- 
quently towards the close of the volcano’s history, when that communication 
had become choked up. But in the case of the volcanic hills to the north of 
Burntisland, the intrusive sheets lie not below but above the interbedded 
lavas and tuffs, through which they come as irregularly as through ordinary 
sedimentary rocks. The most direct connection between volcanic pipes and 
large doleritic and basaltic sheets is to be seen in the east of Fife. To the 
north-east of Largo, for example, a plateau of this nature runs for a distance of 
four miles, with a breadth of about one mile. Round its margin are scattered 
upwards of a dozen small volcanic vents filled with tuff, and some of them 
partially with basalt. Its external characters exactly resemble those of many 
of the true intrusive sheets further west. But as no rock is found lying above 
it, it cannot certainly be affirmed to be itself an intrusive mass, though its 
internal structure, as revealed by the microscope, so entirely agrees with that of 
Se a 
