Through its substance numerous kernels of more glassy lustre are dispersed, each of which usually contains one or more amygdales of dull 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,[166] and some of those from the West of Scotland have been investigated by Professors Judd and Cole.[167] 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, with or without olivine, and usually with more or less interstitial matter. [Footnote 166: _Quart. Journ. Geol. Soc._ vol. xl. (1884).] [Footnote 167: _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.]