42 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



in the north of England have been studied by Mr Teall,* and some of those from 

 the west of Scotland have been investigated by Professor Judd and Mr Cole.1' Taken 

 as a whole, the rocks composing the dykes are found, when examined microscopically, 

 to consist essentially of mixtures of a plagioclase felspar, pyroxene, an iron oxide, and 

 sometimes olivine, usually with more or less interstitial matter. 



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 Fouqtje found the larger porphyritic felspars 

 were mainly labradorite, but included also anorthite, while those of the ground-mass 

 were microliths of albite and oligoclase4 The large felspars scattered porphyritically 

 through the ground-mass are evidently the result of an early consolidation. They 

 are often cracked, and penetrated by the ground-mass, or even broken into fragments. 

 They also include portions of the ground-mass, and present the zonal growth 

 structure in great perfection. The small felspars of the ground-mass, on the other 

 hand, are as obviously the result of a later crystallisation, 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 microliths. Those in the marginal parts of the dyke occur chiefly in the form 

 of these microliths. Curious skeleton forms, composed of aggregates of microliths, 

 connect the latter with the more completely developed crystals, and illustrate the mode of 

 crystallisation of the felspathic constituents of the dykes. § 



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 microliths, 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 many of the dykes, and no serpentinous matter remains to indicate 

 that it was ever present in them. But it is also to be met with in numerous cases, 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 

 " viridite " and " opacite," 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 ground-mass. In others this matter amounts to a half or more of 

 the whole composition, and from such cases we may trace a series of gradations until 



* Quart. Jour. Geol. Soc, vol. xl. (1884). 



t 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. See postea, p. 77, note. % Santorin et ses Eruptions, 1879 p. 203. 



§ See Mr Teall's excellent description of the Cleveland dyke, in the paper above cited. 



