﻿500 Rev. T. G. Bonney — Pitchstones and Fehites of Arran. 



neglected, I determined to re-examine these cases. This I have done 

 on occasion of short visits to Arran in the past and present year, 

 during which I have obtained materials which will, I think, further 

 illustrate this, and throw some light on other rock structures. The 

 points, then, upon which I shall touch in the present paper are the 

 relation (I.) of fissile and banded structures; (11.) of banded, 

 spherulitic, and perlitic structures. 



I. — The Belation of Fissile and Banded Structures. 



I may commence by an additional instance of platy and joint 

 structures. The great quartz- f el site ^ dyke on the Corriegills shore 

 south of Brodick aifords an excellent example. The outcrop of the 

 dyke on the shore is about twenty yards wide, which probably cor- 

 responds to a vertical thickness of nearly ten yards ; the base of the 

 felsite for about eight feet is traversed by a series of platy joints, 

 parallel to its surface, so that the rock splits up into tile-like pieces ; 

 above this a rude vertical prismatic jointing predominates. The 

 upper part again is platy for about six feet, and just at the junction 

 the jointing is very irregular, the platy structure seeming at one 

 place to change for a short distance into a direction at right angles 

 to the usual one. This is caused by the sudden predominance of a 

 set of vertical joints. On following up the central part of the dyke, 

 the joints curve considerably, as though they formed parts of the 

 faces of large irregular spheroids. A basalt dyke breaks through 

 between the base of the above felsite and the sandstone. It is, how- 

 ever, evident that the platy structure is not a true cleavage, due to 

 the pressure of the basalt, because it has evidently been disturbed by 

 the intruding dyke.^ Hence it seems due to the mass cooling rapidly 

 but rather uniformly. 



Great Pitchstone Vein, Corriegills shore. — This rock is rudely 

 columnar, but exhibits a predominating fissile structure top and 

 bottom, best developed at the latter, as well as a very faintly 

 indicated banded structure often only visible on weathered surfaces, 

 likewise parallel to the above surfaces, and perhaps most distinct 

 near them. We find, however, occasional distinct traces of a local 

 fissile structure in the columns, parallel to their faces, and some- 

 times the latter structure curves round and passes into the former. 



Lower Pitchstone, Dunfion. — This dyke well exhibits banded and 

 various fissile structures ; the former being most marked in the 

 eastern part, the latter in the western ; here the laminse run parallel 

 to the faces of long ellipsoidal joint- surfaces. These at one place 

 are crossed and partly obliterated by a set of wavy joints, so that 

 the rock is cut up into small rhomboidal pieces. Where the fissile 

 structure is best developed, there the cross-joints are least. Towards 

 the eastern end the divisional planes of the fissile structure curve 

 round, and really form parts of flattened spheroidal surfaces, and in 

 so doing cut the banded structure at high angles (Fig. 1). 



1 Mentioned by Bryce, Geology of Arran, p. 72. 



^ The base of the felsite near the basalt has a vitrified appearance and has a faint 

 indication of banding. 



