CHAP. XXXV 
INTERSECTING DYKES 
159 
example may be culled from the shore of Skye, near Broadford, where 
the gently-inclined sheets of Lias limestone are traversed by three systems 
of dykes (Fig. 253). One of these systems runs in a N.W. or N.X.W. 
Fig. 252. — Basalt Veins traversing 
bedded dolerites, Kildonan, 
Eigg. 
Fig. 253. — Ground-plan of intersecting Dykes in 
lias limestone, Shore, Harrabol, East of 
Broadford, Skye. 
direction, a second follows a more nearly easterly trend, while the third 
and youngest runs nearly north and south. 
1 6. DYKES OF MOKE THAN ONE IN-FILLING 
The intersections of dykes prove that the process of Assuring in the 
earth’s crust took place at more than one period, and prepare us for the 
reception of evidence that the same line of fissure might be again re-opened, 
even after it had been filled with molten material. Numerous instances 
have now been accumulated in which dykes are not single or simple intru- 
sions, but where the original dyke-fissure lias been re-opened and has been 
invaded by successive uprisings of lava. 1 Compound dykes have thus 
been formed, consisting of two or more parallel bands of similar or dissimilar 
rock. 
While it is not difficult to conceive of the re-opening of a vertical 
fissure during terrestrial strain, and the injection into it of later intrusions 
of a volcanic magma, it is not so easy to understand the mechanism where 
the line of weakness has been slightly inclined or horizontal, and where, con- 
sequently, there has been the enormous superincumbent pressure of the 
overlying part of the earth’s crust to overcome. Yet gently inclined com- 
pound dykes exhibit their parallel bands with hardly less regularity than do 
those that are vertical. The difficulty of explanation is felt most strongly 
in the attempt to realize the origin of the compound sills described in 
Chapter xlviii. 
In the re-opening of dyke-fissures the later intrusions have generally 
1 See an example figured by Maccullocli, Western Isles, plate xviii. Fig. 1. 
