(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.
[166] Quart. Journ. Geol. Soc. vol. xl. (1884).
[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.
The felspar appears to be in some cases labradorite, in others anorthite, but there may be a mingling of several species in many of the dykes, as in the augite-andesite of the Santorin eruption in 1866, wherein Professor Fouqué found that the larger porphyritic felspars were mainly labradorite, but partly anorthite, while those of the groundmass were microlites of albite and oligoclase.[168] The large felspars scattered porphyritically through the groundmass are evidently the result of an early consolidation, unless where they are survivals from fragments of older porphyritic rocks which have been enveloped and partially dissolved in the dykes. They are often cracked, penetrated by the groundmass, or even broken into fragments, and have corroded borders. They sometimes include portions of the groundmass, and present the zonal growth structure in great perfection. The small felspars of the groundmass, on the other hand, are as obviously the result of a later crystallization, for they vary in size and crystallographic development according to their position in the dyke. Those from the centre are often in well-formed crystals, which sometimes pass round their borders into acicular microlites. Those in the marginal parts of the dyke occur chiefly in the form of these microlites, forming the felted aggregate so characteristic of the andesites. Curious skeleton forms, composed of aggregates of microlites, connect the latter with the more completely developed crystals, and illustrate the mode of crystallization of the felspathic constituents of the dykes.[169]
[168] Santorin et ses Éruptions, 1879, p. 203.
[169] See Mr. Teall's excellent description of the Cleveland dyke, in the paper above cited.
The pyroxene is probably in most cases monoclinic (black or common augite), but is sometimes rhombic (usually enstatite, less frequently perhaps hypersthene). It occurs in (a) well-developed crystals, (b) crystalline masses with some of the faces of the crystals developed, (c) granular aggregates which polarise in one plane, (d) separate granules and microscopic microlites, which may be spherical (globulites) or oblong (longulites).
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.
| 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] |