126 
THE TERTIARY VOLCANOES 
BOOK VIII 
branch or send out veins. The younger dykes, on tire other hand, as will be 
more particularly noticed in the following chapter, ai'e distinguished by the 
frequent and remarkable character of their porpliyritic inclusions, by the 
presence of foreign fragments in them, by the greater perfection of their 
jointing, and by their seldom departing much from the vertical. 1 They are 
likewise often markedly acid in composition, including such rocks as 
granophyre, felsite and pitchstone. 
(1) External Characters . — As regards the grain of the rock, every 
gradation may be found, from a coarsely crystalline mass, in which the 
component minerals are distinctly traceable with the naked eye, to a black 
lustrous basalt-glass. Each dyke generally preserves the same character 
throughout its extent. As a rule, broad and long dykes are coarser in grain 
than narrow and short ones. For the most part, there runs along each 
side of a dyke a selvage of finer grain than the rest of the mass. This 
marginal strip varies in breadth from an inch or less up to a foot or more, 
and obviously owes its origin to the more rapid chilling of the molten rock 
along the walls of the fissure. It usually shades away inperceptibly into 
the larger-grained inner portion. Even with the naked eye its component 
materials can be seen to be more finely crystalline than the rest of the dyke, 
though where dispersed porpliyritic felspars occur they are as large in 
the marginal strip as in any other part of a dyke, for they belong to an 
earlier period of crystallization than the smaller felspars of the ground-mass 
and were already floating in the magma while it was still in a molten state. 
This finer-grained external baud, so distinctive of an eruptive and 
injected rock, is of great service in enabling us to trace dykes when they 
traverse other dykes or masses of igneous rock of similar characters to their 
own. When one dyke crosses another, that which has its marginal 
band of finer grain unbroken must obviously be the younger of the two. 
But in many examples in the south of 
Scotland, Argyleshire and the Inner Hebrides, 
the fineness of grain of the outer band cul- 
minates in a perfect volcanic glass. Where this 
occurs, the glass is usually jet black, more rarely 
greenish or bluish black in tint, and varies in 
thickness from about a couple of inches to a 
mere varnish-like film on the outer face of the 
dyke, the average width being probably less 
than a quarter of an inch (Fig. 235). On 
their weathered surface these external glassy 
layers generally present a pattern of rounded or 
Piau of basalt-veins with polygonal prominences, varying up to four or 
five lines or even more in diameter, and separated 
by depressions or narrow ribs. The transition 
from the glass to the crystalline part of the marginal fine-grained strip is 
1 In the Blath Bheinn grouj) of gabbro-hills, however, it is the youngest dykes which have 
been found by Mr. Harker to possess the lowest hade. 
Fig. 235. 
selvages of black basalt-glass, east 
side of Beilin Tiglie, Isle of Eigg. 
