THE TERTIARY VOLCANOES 
BOOK VIII 
partly of the tholeiite type. I have therefore little doubt that these re- 
markable appendages to this dyke are truly offshoots from it, and are not 
to be classed with the general mass of the sills of Central Scotland, which 
are of Carboniferous, partly of Permian, age. The accompanying diagram- 
matic section (Fig. 250) explains the geological structure of the ground. 
An interesting and important fact remains to be stated in connection 
with these sheets. They are traversed by some of the other east and west 
dykes. This is particularly observable in the case of the sheet which 
extends northwards from the dyke through the parish of Torphichen. Two 
well-marked dykes can he seen running westwards among the ridges of the 
sheet. It is obvious, therefore that these particular dykes are younger than 
the sheet. But, as will be shown in the sequel, there is abundant evidence that 
all the dykes of a district are not of one eruption. The intersection of one 
eruptive mass by another does not necessarily imply any long interval of time 
between them. They mark successive, but it may be rapidly successive, 
manifestations of volcanic action. Hence the cutting of the sheets by other 
dykes does not invalidate the identification of these sheets as extravasations 
from the great dyke by which they are bounded. 
15. INTERSECTION OF DYKES 
Innumerable instances may be cited, where one dyke, or one set of dykes, 
cuts across another. To some of these I shall refer in discussing the data 
for estimating the relative ages of 
dykes. In considering the intersection 
from the point of view of geological 
structure, we are struck with the clean 
sharp way in which it so generally 
takes place. The rents into which the 
younger dykes have been injected seem, 
as a rule, not to have been sensibly 
influenced in width and direction by 
the older dykes, but go right across 
them. Hence the younger dykes re- 
tain their usual breadth and trend (Fig. 
251). In trying to ascertain the 
relative ages of such dykes we obtain a 
valuable clue in studying the respective 
“ chilled edges ” of the two intersecting 
masses, as has already been pointed 
out. 
Hot only do dykes cross each 
other, but still more is this the case 
among the narrower tortuous intrusions 
known as Veins (Fig. 252). Among the illustrations which the dykes 
of the Inner Hebrides supply of these features one further characteristic 
Fig. 251. — Intersection of Dykes in bedded 
basalt, Calliach Point, Mull. 
