452 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
between the same strata. They may lie observed to steal across or break 
through the beds, so as to lie successively between difierent layers, iso 
more instructive example of this relation on a small scale could be cited 
than that of the intrusive sheet which has 
lieen laid open in the Dodhead Limestone 
Quarry, near Burntisland. As shown in the 
accompanying figure (Fig. 163), this rock 
breaks tlirough the limestone and then spreads 
out among the overlying shales, across whicli 
it passes obli(][uely. 
Among the larger sills this transgressive 
character is seen to be sometimes manifested 
on a great scale. Thus, along the important 
licit of intrusive rocks tliat runs from Kilsyth 
to iStirling, the Hurlet Limestone lies in one 
„ . _ . , . . , place below, in another above, the invading 
cio.siiig and seudiiig threads into m^ss, blit 111 the intervening grouiid has been 
engulphed in it. Similar eviileuce of the 
widely separate horizons occupied by different 
parts of the same sill is supplied at Kilsyth, wliere the intrusive sheet 
lies about 70 or 80 fathoms below the Index Limestone, while at Croy, in 
the same neighbourhood, it actually passes above that seam.^ 
Other interesting evidence of the intrusive nature of the Carboniferous 
dolerite sills of Central Bcotland is supplied by the internal modifications 
which the eruptive rock has undergone by contact with the strata between 
which it has been thrust. These alterations, thougli partlj- visible to the 
portions of sliale, Salisbury Crags, 
Eiliiihurgli. 
Fig. 163. — Intrusive sheet invading limestone and sfiale, Dodhead Quarry, near Burntisland. 
naked eye, are best studied in thin slices with the aid of the microscope. 
Tracing the variations of an intrusive dolerite outwards in the direction of 
the rocks which it has invaded, we perceive change first in the augite. The 
large crystals and kernels of that mineral grow smaller until they pass into 
a granulated form like that characteristic of basalts. Tlie large plates and 
^ Explanation of Sheet 31, Geological Survey of Scotland, §§ 43 and 83. 
