i6o 
THE TERTIARY VOLCANOES 
BOOK VIII 
taken place along the walls, or where the dykes were already compound, 
between some of the component hands. Less frequently the first dyke lias 
been split open along the middle, and a second injection has forced its way 
along the rent. 
Of the first of these two types, numerous instances have now been 
observed in the West of Scotland. If the portion of a compound 
dyke exposed at the surface be limited in extent, we may be unable to 
determine which is the older of two parallel bands of igneous rock, 
though the fact that they present to each other the usual fine-grained edge 
due to more rapid cooling, shows that they are not one but two dykes, 
belonging to distinct eruptions. So far as I have noticed, where one of the 
dykes can be continuously traced for a considerable distance, the other is 
comparatively short. I infer that the shorter one is the younger of 
the two. 
In the Strath district of Skye, Mr. Harker has recently observed that 
many of the basic dykes, both those older and those younger than the 
granophyre protrusions, are double, triple or multiple. Thus in a con- 
spicuous dyke, more than 100 feet wide, to the south-east of Loch Kilchrist, 
belonging to the older series, he has detected at least six contiguous dykes 
which as they are traced south-eastward, in spite of their interruption by 
the Beinn an Dubhaich granite, can be seen to separate and take different 
courses, or successively die out. He remarks, further, that “ many cases of 
apparent bifurcation of dykes are really due to the separation of distinct 
dykes which have run for some distance in one fissure. Sometimes apparent 
variations in the width of a dyke are to be explained by this dying out of 
one member of a double dyke. These multiple dykes are less easily de- 
tected in the newer than the older set, owing to greater uniformity of 
lithological type in the prevalent kinds and to the frequent absence of 
chilled selvages.” 1 An example of a compound basic dyke cutting the crest 
of the gabbro-mass of the Cuillin Hills is shown in Fig. 333. 
Instances of the second type of compound dykes are less common. Here, 
instead of being reopened along one of the walls, the fissure has been ruptured 
along the centre of the dyke, and a second injection of molten material has 
then taken place. This structure may be observed where the materials of the 
compound dyke are on the whole similar, such as varieties of dolerite, basalt, 
diabase or andesite. In these cases the rock of the central dyke is generally 
rather fine-grained, sometimes decidedly porphyritic, and often a true basalt. 
Where broad enough to show the difference of texture between margin and 
centre, it exhibits the usual close grain along its edges, indicative of quicker 
cooling. The older dyke presenting no such change at its junction with 
the younger, was obviously already cooled and consolidated before its rupture. 
Whilst the centre of a dyke has occasionally proved to be a line of 
weakness which has given way under intense strains in the terrestrial crust, 
this rupture and the accompanying or subsequent ascent of molten material 
in the reopened fissure may sometimes have been included as phases of one 
1 MS. notes supplied ty Mr Harker. 
