﻿Rev. T. G. Bonney — Pltchstones and Felsites of Arran. 503 



will show that though this may often be the case, the mechanical 

 forces produced by contraction have also sometimes an influence 

 upon it, and may even determine whether or not this structure shall 

 be set up. 



The first case which I shall quote is not from Arran, but is ex- 

 hibited in a slide of liparite from the Lipari Islands. It is a por- 

 phyritic rock containing many crystals of felspar, with a little quartz, 

 biotite, and iron-peroxide. An ordinary lens shows the dull white 

 ground-mass to be traversed by a series of more transparent lines 

 dividing it up into polygons, so that it looks like a miniature represen- 

 tation of a columnar -jointed surface. Inside the polygons, the rock 

 exhibits a faint radial structure. Under the microscope these cracks 

 appear to have been true minute divisional surfaces, though they now 

 seem to be closed by a generally paler mineral deposit, which is 

 dark between crossed prisms. Sometimes also we see a thin dark 

 line, bordered on each side by a clear space. The interior of the 

 polygons commonly exhibits very clearly the usual fibrous radial 

 structure ; ^ but in some of the smaller it is very ill-defined. In 

 some cases we note a kind of circle inscribed in the polygon, within 

 which the radial structure is more marked ; generally there is no 

 nucleus visible in the spheroids, though now and then they seem 

 to become rather more crystalline on approaching the centre ; nor do 

 they appear to have any very immediate relations with the larger 

 embedded crystals (compare Fig. 3). 



Passing next to a perlitic pitchstone from Meissen, we observe 

 that it exhibits a flow-like structure, indicated by wavy bands (in 

 outline like some cirrus clouds), produced by innumerable very 

 minute opaque microliths. Through these the perlitic structure 

 cuts with perfect independence. There are no spherulites to be seen 

 in the slide. A perlite, also, from Glashiitte (Schemnitz), shows very 

 well this wavy flow-structure; but here the bands are so conspicuous 

 and clearly marked that the rock looks like a miniature model of a 

 contorted gneiss. A few spherulites occur in this, which in most 

 cases, though not in all, seem connected with the larger crystals 

 occurring in the matrix. 



The great pitchstone vein on the Corriegills shore exhibits, as has 

 been said, faint indications of a banded structure. Under the micro- 

 scope it appears full of a microlithic dust with larger belonites 

 singly or in groups, surrounded by slightly clearer spaces ; a good 

 many of these belonites are roughly parallel. The smaller vein has also 

 indications of banding, though slighter, with a similar microscopic 

 structure : but the clearer spaces are larger, and themseh^es indicate 

 a kind of banding. Tliere are one or two spherulites surrounded by a 

 clearer ring. More distinct bauds are seen in the lower pitchstone vein 

 of Dundhu, and some of the dykes of the Tormore shore, the micro- 

 scopic structure of which is excellently depicted by Mr. S. Allport,- 

 who says (p. 6), "The rock is simply a continuous homogeneous 

 glass, in which the de vitrified or crystallized particles have arranged 



1 See Mr. S. AUport, Geol. Mag. Dec. I. Vol. IX. p. 541. 



2 Geol. Mas. Lee. I. Vol. IX. PI. I. Fig. 2. 



