500 Rev. T. G. Bonney — Pitchstones and Felsites of Arran. 
neglected, I determined to re-examine tbese 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; (II.) of banded, 
spherulitic, and perlitic structures. 
I . — The Relation of Fissile and Banded Structures. 
I may commence by an additional instance of platy and joint 
structures. The great quartz-felsite 1 dyke on the Corriegills shore 
south of Brodick affords an excellent example. The outcrop of the 
dyke on the shore is about twenty yards wide, which probably eor- 
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 rüde vertical prismatic jointing predominates. The 
upper part again is platy for about six feet, and just at the junction 
the jointing is very irregulär, 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 irregulär 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. 2 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 predoiuinating fissile structure top and 
bottom, best developed at the latter, as well as a very faintly 
indicated banded structure often only visible on weathered snrfaces, 
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- 
tiines the latter structure curves round and passes into the foraier. 
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 laminae 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. 
2 The base of the felsite near the basalt has a vitrified appearance and has a faint 
indication of banding. 
