136 
THE TERTIARY VOLCANOES 
BOOK VIII 
followed inwards until in the central parts of a broad dyke little or no trace 
of the interstitial matter may be left. 
The most instructive example of the process of devitrification which has 
come under my observation occurs in the Eskdale dyke. The central 
present a true glass, which in thin sections is 
perfectly transparent and almost colourless, 
but by streaks and curving lines of darker tint 
shows beautiful flow-structure. The devitri- 
fication of this glass has been accomplished by 
the development of crystallites and crystals, 
which increase in number until all the 
vitreous part of the rock disappears. What 
seems under a low power to be a structureless 
or slightly dusty glass can be resolved with a 
higher objective into an aggregate of minute 
globules or granules (globulites), which average 
perhaps ^oWtr °f an i nc h diameter. 
Some of these bodies are elongated and even 
dichotomous at the ends. These granules are 
especially crowded upon clear yellow dart-sliaped 
rods, which in turn are especially prominent 
upon crystals and crystalline grains of augite 
that bristle with them, while the immediately 
surrounding glass has become clear. There 
can be little doubt that these rudimentary 
bodies are stages in the arrested development of augite crystals. There 
occur also opaque grains, rods and trichites, which no doubt consist in 
whole of magnetite (or other iron oxide), or are crusted over with that 
mineral. 
At least two broad types of microscopic structure may be recognized 
among the basic and intermediate dykes. (1) Holocrystalline, or with only 
a trifling proportion of interstitial matter. This type includes the dolerites 
and basalts, as well as rocks which German petrographers would class as 
diabases or diabase-porpbyrites. The rocks are very generally characterized 
by ophitic structure, where the lath-shaped felspars penetrate the augite, 
and are therefore of an earlier consolidation. In such cases there is a 
general absence of any true interstitial matter. The rocks of this type are 
often rich in olivine, and appear to be on the whole considerably more basic 
than those of the second group. It is observable that they increase in 
numbers from the centre of Scotland westwards, and throughout the region 
of the basalt-plateaux they form the prevailing type. (2) In this type 
there is a marked proportion of interstitial substance, which is inserted in 
wedge-shaped portions among the crystallised constituents (“ intersertal 
structure ” of Kosenbusch). The ophitic structure appears to be absent, 
and olivine is either extremely rare or does not occur at all. The rocks of 
1 Proc. Roy. Phys. Soc. Edin. v. (1880), p. 255. 
cores ” already referred to 
Fig. 240. — Microscopic .structure of tlie 
vitreous part of the Eskdale Dyke, 
This section shows a crystal of augite, en- 
closing magnetite and surrounded with 
microlites, each ofwliioh consists of a cen- 
tral pale yellow rod crusted with pale 
yellow isotropic globulites. The glass 
around this aggregation is clear, but at a 
little distance globulites (many of them 
elongated and dichotomous) abound, 
with here ancl there scattered microlites, 
some of which arc curved and spiral. 
(S00 diameters.) 1 
