﻿504 Rev. T. G. Bonney — Pitchstones and Felsites of Arran. 



themselves in alternate layers characterized by differences in texture 

 only." The southern dyke at Tormore seems to indicate that the 

 banding is here caused by the more complete crystallization of the 

 augitic constituent, the fine dust having almost disappeared from the 

 glass with a marked increase in the number of belonites ; there are 

 also several spherules. The pitchstone by the path in Birk Glen 

 shows some fairly regular isolated bands of yellowish glass, together 

 with several more irregular patches. In no case, however, have the 

 ends of these a torn or " teazed " look, as if they had been subjected 

 to strain ; some indeed are quite round. Belonites occur in both the 

 clear and the yellow glass, sometimes singly, but commonly with 

 the usual alga-like aggregates. They pass from the clear to the 

 yellow glass, sometimes, however, being a little crowded on the 

 surface of the latter, as though they stuck in a denser fluid. In it 

 also the lateral microliths are less clearly defined, as though the side 

 branches had' fallen from the main stem of the plant, and had been 

 replaced by a lichen growth. "With polarized light these bands 

 show an imperfect spherulitic structure, as do some of the rounded 

 spots. In the former, however, there is no indication of any dis- 

 turbance of the spherulitic structure, that is, it does not appear as if 

 one or more spherules had been elongated into a band, but as if a 

 line of imperfect spherules had formed out of a band. 



The first pitchstone on the shore south of King's Cove gives 

 indications of a rough perlitic structure. Under the microscope this 

 appears more as a polygonal network of exceedingly minute cracks 

 than as the ordinary perlitic structure. There are irregular bands 

 and cloudy spots of yellow glass ; the latter frequently showing 

 rings of darker glass, which, on examination, prove to be roughly 

 concentric with the above reticulations. With crossed prisms these 

 rings prove to be the boundaries of more or less perfect spherules. 

 This rock does not show the alga-like tufts, but has numerous 

 scattered belonites about -001 inch long. These often pass indiffer- 

 ently into the spherulites, but are sometimes arranged radiately. 

 There are a good many larger crystals scattered about, chiefly of 

 sanidine ; some of which (but not all) show a growth of belonites 

 perpendicular to their surface, and a radial structure of the sur- 

 rounding mass. A pitchstone, from an erratic,^ on the Old Lamlash 

 Road, also shows a similar spherulitic growth on some of the larger 

 crystals contained, but here also this structure is visible in many 

 isolated patches of brown glass. 



The larger crystals occurring in the pitchstones sometimes, as 

 described by Mr. Allport, include portions of the glassy matrix, but 

 rarely, if ever, well-defined belonites. Sometimes they have evidently 

 been broken, and the fragments lie close together. They appear then 

 to be the first things formed in the rock, and perhaps may some- 

 times be very much anterior to the solidification of the ground-mass.^ 



^ I am not aware that the locality where this occurs in situ is known. It is the 

 same rock as is descrihed by Mr. Allport from a boulder occurring near W. Benan 

 (Geoi,. Mag. Dec. I. Vol. IX. p. 537). 



2 I ha've observed in numerous instances in trachytes, felsites, etc., that the forma- 



