446 
THE TERTIARY VOLCANOES 
BOOK VIII 
They generally show considerable irregularity in breadth and direction, 
sometimes sending out strings into the surrounding rock (Fig. 3 SI). The 
outer portions are not infrequently more glassy and obsidian-like than the 
interior. Occasionally the vitreous character disappears by devitrification, 
and the rock assumes the texture of a compact felsite or of a spherulitic 
rock. 
Among the later movements of the acid magma account must be taken 
here of the pale fine-grained veins which have already been referred to as 
traversing the granophyre bosses. These intrusions, so well seen in the 
bosses of Skye and St. Hilda, are often so close in texture that they may be 
called quartz-felsites. Their sharply-defined edges and felsitic character 
suffice to separate them from what are termed “ veins of segregation.” In 
at least one instance, that of Meall Dearg, already cited, a mass of typical 
granophyre which has developed spherulitic and flow-structures along its 
margin, and which sends out dykes having the very same structures for a 
distance of several hundred feet across the banded gabbros, is itself traversed 
by a dyke of precisely similar character. Here we see that after the intru- 
sion of its apophyses, and after its own consolidation in the upper parts, the 
granophyric magma that rose into rents in the solidified portion retained 
the same tendency to produce large spherulites as it had shown at first. 
The fine felsitic veins that traverse the granophyre of the Eed Hills are 
now being mapped by Mr. Barker during the progress of the Geological 
Survey. He has not yet obtained evidence of the age of these veins in 
relation to the latest basic dykes. He has observed that they appear to be 
on the whole rather less acid than the material of the surrounding bosses, 
though they were probably all connected with the same underlying acid 
magma from which the bosses were protruded. A somewhat similar relation 
has been noticed between older granites and their surrounding dykes, as in 
Cornwall and Galloway. 
