138 
THE TERTIARY VOLCANOES 
BOOK VIII 
these veins is probably orthoelase. It forms a much larger percentage of 
the entire rock than the felspar does in normal dolerites. 
2. Trachyte Dykes. — In the Cowal District of Argyleshire, and 
in the south of Skye, Mr. Clough has encountered a limited number 
of dykes of trachyte. On a hasty inspection these are not always readily 
distinguished from the basalt-dykes with which they agree in general 
external aspect and in direction. Where their relation to these dykes, 
however, can he determined they are found to traverse them, and thus to 
be on the whole later, though one case has been observed where a trachytic 
dyke is in turn traversed by one of the basic series. Mr. Clough has 
supplied me with the following notes of his observations regarding the 
trachytic dykes. They are all characterized by the possession of spherulitic 
structures near their margins. These features, easily perceptible to the 
naked eye, afford the readiest means of distinguishing the dykes of this 
group. So abundant are the spherulites that they not infrequently im- 
pinge on each other in long parallel rows forming rod-like aggregates. 
Thus in a dyke near Craigendavie, at the head of Loch Striven, numerous 
planes about a quarter of an inch apart, and composed of such close-set 
rods, may be observed running parallel to the marginal wall for a distance 
of several inches from the edge. Most of these planes show on their surfaces 
that the rods are always parallel to each other, but may run in different 
directions in the different layers, being sometimes horizontal, sometimes 
vertical, or at any angle between. On examination, each rod is found to 
be made up of polygonal bodies, the angles of which are quite sharp, but 
with their sides often slightly curved, as if they had assumed their forms 
from the mutual pressure of original spherical bulbs. Further scrutiny 
shows that the polygonal bodies often exhibit an internal radiate structure. 
In the central parts of the dyke the spherulitic arrangement is not 
traceable. About a foot from the margin it begins to be recognizable. At 
a distance of three or four inches the spherulites are about the size of peas, 
and gradually diminish towards the edge until they can no longer be seen. 
Another characteristic of the trachyte dykes has been found by Mr. 
Clough to be a useful guide in discriminating them from the basalt-group. 
While the amygdales in the latter are generally rudely spherical, those in 
the trachytes are commonly elongated in the direction of the length of the 
dyke, and are frequently three quarters of an inch, sometimes even an inch 
and a half, in length, though less than a quarter of an inch in breadth. 
A good example of these trachytic dykes, which occurs at Dunans, 
about the head of Glendaruel, has been examined microscopically and 
chemically. The central better crystallised portion was found by Mr. 
Teall to be composed mainly of small lath-shaped crystals of orthoelase, 
together with scales of brown biotite, a few prismatic crystals of pale 
somewhat altered pyroxene and scattered granules of magnetite. The 
chemical analysis of this rock by Mr. J. H. Player gave the following- 
composition : — 
