CHAP. XXXV 
GEOLOGICAL AGE OF THE DYKES 
1 73 
and west direction being comparatively few, while the younger X.N.W. 
series is well developed. The great sheets or “ sills ” connected with one of 
the Stirlingshire dykes, already described, appear to me to furnish similar 
evidence in the younger dykes which run through them. And this 
evidence is peculiarly valuable, for it shows a succession even among 
adjacent dykes which all run in the same general direction. 
But in all these cases it is obvious that we have little indication of the 
length of time that intervened between the successive injections of the 
dykes. In Skye, however, more definite evidence presents itself that the 
interval must have been in some cases a protracted one. As far back as 
the year 185 7, 1 I showed that the basic dykes of Strath in Skye are 
of two ages ; that one set was erupted before the appearance of the 
“ syenite ” (granophyre) of that district, and was cut off by the latter rock ; 
and that the other arose after the “ syenite " which it intersected. 
Recent re-examination has enabled me to confirm and extend this observation. 
The younger series which traverses the granophyre is much less numerous 
than the older series in the same districts. In Chapter xlvi., where the 
relations of the granophyres to other members of the volcanic series will 
be discussed, further details will be given from that region of Skye to 
demonstrate that there is a pre-granophyre and a post-granopliyre series of 
basic dykes. As a good illustration of the younger series I may refer to 
the way in which these rocks make their appearance in the island group of St. 
Kilda, where botli the gabbros and granophyres of the Tertiary volcanic 
series are characteristically developed. Numerous dykes traverse both 
these rocks. Those in the gabbro are more abundant than those in the 
granophyre — a circumstance which is exactly paralleled among the basic and 
acid bosses of Skye. It is not improbable that in these remote islands a 
similar difference in age and in petrographical character may be made out 
between two series of dykes, one older and the other younger than the 
granophyre. There is ample proof, at all events, of a post-granophyre series. 
The pale colour of the precipices in which the St. Kilda granophyre 
plunges into the sea gives special prominence to the dark ribbon-like 
1 Quart. Jour. Geol. Soc. vol. xiv. p. 16. 
