44 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



type. (2) In this type there is a marked proportion of interstitial substance, which is 

 inserted in wedge-shaped portions among the crystallised constituents (" intersertal 

 structure " of Rosenbusch). The ophitic structure appears to be absent, and olivine is 

 either extremely rare or does not occur at all. The rocks of this group are obviously less 

 basic than those of the other. They form the large dykes that rise so conspicuously 

 through the south of Scotland and north of England, and their general characters are well 

 described by Mr Teall in the paper already cited. In some instances they enclose 

 abundant porphyritic felspars of earlier consolidation, and then present most of the 

 characters of andesites. Professor Rosenbusch has recently extended the name of 

 '■' Tholeiites" to rocks of this group in the north of England.* The vitreous condition is 

 found in both types, but is perhaps more frequent in the second. The glass of the 

 basalts, however, even in thin slices, is characteristically opaque from its crowded 

 inclusions ; while that of the andesitic forms, though black in hand specimens, appears 

 perfectly transparent and sometimes even colourless in thin slices. 



3. Gliemical Characters. — The only one of these to which reference will be made here 

 is the varying proportion of silica. While the dykes as a whole are basic, some of them 

 contain so high a percentage of silica as to link them with the acid rocks. The pro- 

 portion of this ingredient ranges from less than 50 to nearly 60 per cent. The rocks 

 with the lower percentage of acid are richer in the heavy bases, and have a specific 

 gravity which sometimes rises above 3*0. They include the true dolerites and basalts. 

 Those, on the other hand, with the higher ratio of silica, are poorer in the heavy bases, 

 and have a specific gravity from 276 to 2*96. They comprise the "tholeiites," 

 "" andesites," and other more coarsely crystalline rocks of the great eastern and south- 

 eastern dykes.t 



Not only do the dykes differ considerably from each other in their relative proportions 

 •of silica, but even the same dyke may be found to present a similar diversity in different 

 parts of its mass. It has long been a familiar fact that the glassy parts of such basic rocks 

 are more acid than the surrounding crystalline portions. The original magma may be 

 regarded as a natural glass or fused silicate, in which all the elements of the rock were 

 dissolved, and which necessarily became more acid as the various basic minerals crystal- 

 lised out of it. J In the Eskdale dyke the silica percentage of this glassy portion is 58*67, 

 that of the little kernels of black glass dispersed through the rock as much as 65'49.§ 

 In the Dunoon dyke observed by Mr Clough the siliceous or jaspideous veins contain no 

 less than 68 "05 per cent, of silica, while the mass of the dyke itself shows on analysis 

 only 47*36 per cent.|| 



* Mikroskopische Physiographic, 2nd edition, 504 et seq. 



t For analyses of dykes, see I. L. Bell, Proc. Roy. Soc, xxiii. p. 546 ; J. S. Grant Wilson, Proc. Roy. Phys. Soc. 

 Edin., v. p. 253 ; Teall, Quart. Jour. Geol. Soc, xl. p. 209 ; Judd and Cole, Quart. Jour. Geol. Soc, xxxix. p. 444. 



X On this subject see a paper by Dr A. Lagorio, " Uber die Natur der Glasbasis sowie der Krystallisationsvorgange 

 im eruptiven Magma," Tschermak's Mineralog. Mittheil., viii. (1887) p. 421. 



§ J. S. Grant Wilson, Proc. Roy. Soc Phys. Edin., v. (1880) p. 253. 



|| Unpublished analyses made by Prof. Dittm.vr of Glasgow, and communicated to me by Mr Clough. 



