CHAP. XLVIII 
THE ACID DYKES AND VEINS 
443 
Broadford and the Sound of Scalpa. Broad dykes of granophyre which 
traverse the Cambrian limestone of that district might be supposed at first 
sight to be cut off by the intrusions of gabbro. But closer examination 
proves that their apparent truncation arises from their suddenly breaking 
up into a network of small veins where they abut against the basic rock. 
This structure evidently belongs to the same type as that of the St. Kilda 
granophyre. 
Compound dykes and sills, where one or more of the injections has con- 
sisted of acid material, have been already noticed as intimately associated 
together in Skye (p. 162). Dykes of this nature are more particularly 
abundant in Strath, especially along its eastern side. In addition to the 
examples cited already from that district, 1 may refer to other two which 
Fig. 380. — Section of intruded veins of various acid rocks above River Olachaig, Mull. 
a a, basalt, dolerite, etc. ; b b, granophyre. 
intersect the Middle Lias shales and limestones in the island of Scalpa. 
They are both compound dykes, but the more basic marginal bands are not 
always continuous, having possibly been here and there dissolved by the 
acid invasion. Though they do not show any distinct spherulitic forms, 
the presence of flow-structure is indicated by the thin slabs into which the 
rocks weather parallel to the dyke-walls. The rock in each case is a fine- 
grained felsitic mass, with bi-pyramidal crystals of quartz. It is observable 
that where these dykes come directly against the Liassic strata, the latter 
are more seriously indurated than where they are traversed by the ordinary 
basic dykes. 
In the central mountainous tract of the island of Mull veins of acid 
material are extraordinarily abundant. They probably proceed from a much 
larger subterranean body of granophyre than any of the comparatively small 
bosses of this rock which appear at the present surface of the ground. They 
