CHAP. XXXIV 
MATERIALS OF THE DYKES 
25 
3. NATURE OF COMPONENT ROCKS 
The Tertiary dykes of Britain include representatives of four distinct 
groups of igneous rocks. 1st, The vast majority of them consist of plagio- 
clase-pyroxene-magnetite rocks with or without olivine. These are the 
normal basalts and dolerites. 2nd, A number of large dykes have a rather 
more acid composition and are classed as andesites. 3rd, A few dykes of 
trachyte have been observed in Cowal and in Skye cutting the dykes of 
basalt (p. 138). 4th, In some districts large numbers of still more acid 
dykes occur. These are sometimes crystalline in structure (granopliyre), 
more frequent!}' felsitic (felsite, spherulitic quartz-porphyry), and often glassy 
(pitchstone). I 11 some exceptional cases the basic and acid materials are 
conjoined in the same dyke. Such compound varieties are described at p. 
161. The acid dykes, connected as they so generally are with the large 
bodies of granophyre or granite, are doubtless younger than the great 
majority of the basic dykes. They will be treated in connection with the 
acid intrusions in Chapter xlviii. 
By far the greater number of the dykes of the Tertiary volcanic 
series belong to the first group, and it is these more especially which 
will be discussed in the present and the following Chapter. As, however, 
the andesitic group is intimately linked with the basaltic it will be here 
included with them. 
1. Basalt, Dolerite and Andesite Dykes.- — To the field-geologist, 
who regards merely their external features, the Tertiary dykes present a 
striking uniformity in general petrographieal character. They vary indeed 
in fineness or coarseness of texture, in the presence or absence of porphyritic 
crystals, amygdales, glassy portions and other points of structure. But there 
is seldom any difficulty in perceiving that they generally belong to one or other 
of the types of the basalts, dolerites, diabases or andesites. This sameness of 
composition, traceable from Yorkshire to Skye and from Donegal to Perth- 
shire, is one of the strongest arguments for referring this system of dykes to 
one geological period. At the same time, there are enough of minor variations 
and local peculiarities to afford abundant exercise for the observing faculties 
alike in the field and in the study, and to offer materials for arriving at some 
positive conclusions regarding the geological processes involved in the uprise 
°f the dykes. 
There appears to be reason to believe that, when the petrography of the 
dykes is more minutely studied, marked differences of material will be 
found to denote distinct periods of eruption. Already Mr. A. Harker of the 
Geological Survey, who is engaged in mapping the interesting and compli- 
cated district of Strath in Skye, has observed that the dykes which are 
older than the great granophyre bosses of that tract may be distinguished 
from those which are later than these protrusions. The older basic dykes 
are not conspicuously porphyritic, are frequently marked by a close-grained 
margin or even with a veneer of basalt-glass, sometimes have an inclination 
of as much as 45°, are occasionally discontinuous, and not infrequently 
