134 
THE TERTIARY VOLCANOES ' 
BOOK VIII 
core and an outer more lithoid sheath. By the coherence of their polygonal 
and irregular faces, and the greater durability of their material, these 
sheaths project on the weathered wall of the vitreous centre of the dyke in 
a curiously reticulated grouping of prominent ribs each about two inches 
broad (Big. 239, A), while the vitreous cores, being more readily acted on by 
the weather, are hollowed out into little cup-shaped depressions. Each rib is 
thus composed of the sheaths or outer lithoid portions of two prisms, the line 
of separation being marked by a suture along the centre (B). Between this 
median suture and the inner glassy core the rib is further cut into small 
segments by a set of close joints, which are placed generally at right angles 
to the course of the rib (C). Examined with a lens, the lithoid substance of 
these sheaths has a dull finely granular aspect, like that of felsitic rocks, 
with scattered felspars. It is obviously a more devitrified condition of the 
material which forms the core of each prism. This material presents on a 
fresh fracture a deep iron-black colour, dull resinous lustre and vitreous text- 
ure. It at once recalls the aspect of many acid pitchstones, and in the early 
days of petrography was naturally mistaken for one of these rocks. Through 
its substance numerous kernels of more glassy lustre are dispersed, each of 
which usually contains one or more amygdales of chill white chalcedony, but 
sometimes only an empty black cavity. These black glistening kernels of 
glass, of all sizes up to that of a small bean, scattered through the dull 
resinous matrix, form with the white amygdales the most prominent feature 
in the cores; but crystals of felspars may also be observed. Some details 
of the microscopic characters of this remarkable structure will be given in a 
subsequent page. The relation of the cores and sheaths to the prismatic 
jointing of the rock seems to show that devitrification had not been 
completed when these joints were established, and that it proceeded from 
the faces of each prism inwards. 
(2) Microscopic Characters . — Much information has now been obtained 
regarding the microscopic structure of the basaltic, doleritic and andesitic 
dykes. The crystalline characters of those in the North of England have 
been studied by Mr. Teall, 1 and some of those from the West of Scotland 
have been investigated by Professors Judd and Cole. 2 Taken as a whole, 
the rocks composing the dykes are found, when examined microscopically, 
to consist essentially of mixtures of a plagioclase felspar, pyroxene and iron 
oxide, witli or without olivine, and usually with more or less interstitial matter. 
The felspar appears to be in some cases labradorite, in others anortliite, 
but there may be a mingling of several species in many of the dykes, as in 
the augite-andesite of the Santorin eruption in 1 8 6 6, wherein Professor Pouque 
found that the larger porpliyritic felspars were mainly labradorite, but partly 
anorthite, while those of the groundmass were microlites of albite and 
oligoclase. 3 The large felspars scattered porphyritically through the ground- 
1 Quart. Journ. Geol. Soc. vol. xl. (1884). 
2 Op. cit. vol. xxxix. (1883) p. 444 (basalt-glass) ; xlii. (1886) p. 49, where Professor Judd 
discusses the gabbros, dolerites and basalts as a whole. 
3 Santorin et ses Eruptions, 1879, p. 203. 
