CHA1'. XLVIII 
THE ACID DYKES AND VEINS 
439 
devitrification flowing round the spherulites and any enclosed fragments as 
perfectly as in any rhyolitic lava (Fig. 378). 
In regard to their modes of occurrence, the dykes of acid material differ 
in some important respects from those of basic composition. More especially 
they are apt to assume the irregular venous form, rather than the vertical 
wall-like character of ordinary dykes. They take the form of dykes, 
particularly where their material has been guided in its uprise by one or 
more already existent basic or intermediate dykes, as in the compound dykes, 
already described. The conditions for their production must thus have been 
essentially different from those of the great body of the basic dykes. Their 
intrusion was not marked by any general and widespread Assuring of the 
earth’s crust, such as prepared rents for the reception of the basalt and an- 
desite dykes. They were rather ■<,»*»*«««*» ■, 
W ii M fl/;; (MU' 
f I j 1 7 / / A ' \\\ i 
l i f! \ 
accompaniments of the protru- 
sion of large masses of acid 
magma into the terrestrial crust. 
This magma, as we have seen, 
was often markedly liquid, and 
was impelled, sometimes with 
what might be called explosive 
violence, into the irregular cracks 
of the shattered surrounding 
rocks or into pre-existing dyke- 
fissures. Hence long straight 
dykes of the acid rocks are much 
less common than short irregular 
tortuous veins and strings. 
Much difference may be 
noticed among the granophyre 
bosses in regard to their giving 
off a fringe of apophyses. Thus, 
along the well-exposed boundary 
of Beinn-an-Dubhaich in Skye, 
though the edge of the boss is 
remarkably notched, hardly any 
veins deserving the name diverge 
from it. On the other hand, the 
ridge of Meall Dearg at the head 
of Glen Sligaehan, already re- 
ferred to, is distinguished by the 
number and variety of the dykes 
and veins which proceed from the granophyre and traverse the banded 
gabbros. As this locality has been elsewhere fully described, I will give here 
only the leading structural features which it presents. 1 
1 Professor Judd {Quart. Journ. Geol. Soc. vol. xlix. (1893), p. 175) described tlie granophyre 
dykes of this locality as inclusions of Tertiary granite in the gabbro, and cited them in proof of 
SeecCe of 
Fig. 376. — Plan of portion of the ridge north of Druim 
an Eidhne, Glen Sligaehan, Skye, showing three dykes 
issuing from a mass of granophyre. 
a , gabbros; b, granophyre; I. 11. ILL, three dykes proceeding 
from the granophyre. The arrows show the direction of dip 
of the bands of gabbro. 
