The black iron-oxide is sometimes magnetite, sometimes ilmenite, or other titaniferous ore. Apatite not infrequently occurs among the original constituents. Olivine is entirely absent from most of the large solitary dykes, especially at a distance from the great volcanic centres, and no serpentinous matter remains to indicate that it was ever present in them. But it is to be met with in numerous basalt-dykes in the volcanic areas, either in sparsely scattered or in tolerably abundant crystals. Biotite occasionally appears. Among the secondary products, calcite and pyrites are doubtless the most common. To these must be added quartz, chalcedony and various zeolitic substances, besides the aggregates which result from the decomposition of the ferro-magnesian constituents and the oxidation of the ferrous oxides. In many dykes there is little or no interstitial matter between the crystalline constituents of the groundmass. In others this matter amounts to a half or more of the whole composition, and from such cases a series of gradations may be traced into a complete glass containing only the rudimentary forms of crystals (globulites, longulites, etc.), with scattered porphyritic crystals of an earlier consolidation. The process of the disappearance of this original glass may be admirably studied in many dykes. At the outer wall, the glass remains nearly as it was when contact with the cold walls of the fissure solidified it. From that external vitreous layer the successive devitrification products and crystalline growths may be followed inwards until in the central parts of a broad dyke little or no trace of the interstitial matter may be left. [Illustration: Fig. 240.--Microscopic structure of the vitreous part of the Eskdale Dyke. This section shows a crystal of augite, enclosing magnetite and surrounded with microlites, each of which consists of a central 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 and there scattered microlites, some of which are curved and spiral. (800 diameters.)[170] ] [Footnote 170: _Proc. Roy. Phys. Soc. Edin._ v. (1880), p. 255.]