﻿506 Mev. T. G. Bonney — Pitchstones and Felsites of Arran. 



described by Mr. S. Allport/ namely, a felsitic structiTre outside the 

 circle, and a fibrous radial structure inside, the division between 

 them not being quite so sharp or regular as it appeared before. By 

 searching about the slide we can find almost every stage, from the 

 setting up of the fibrous radial structure at a point in the matrix, to 

 a spherule with a well-defined circular boundary. There is no 

 evidence that a nucleus of a different mineral character is needed 

 for the formation of a spherule. 



Fig. 3. — Spherulitic Felsite. Diagrammatic sketch from microscope. The radial 

 structure in the spheriilites is rather exaggerated ; the exterior granulated part with 

 both prisms shows ' felsitic ' structure, sometimes with slight radial tendency, the 

 polygons are defined by opaque clotted (ferruginous ?) dust. 



The next rock to be examined is the spherulitic felsite on the 

 Corriegills shore. It has been noticed by Macculloch and others, and 

 its microscopic structure has been excellently described by Mr. S. 

 Allport; but still its relations to the neighbouring rocks do not 

 seem to have been fully worked out, so that I must describe it at 

 some length, as these seem to throw much light on the formation of 

 some of the above structures. A compact felsite dyke about 12 

 yards across is exposed on the shore, and can afterwards be traced 

 running up the inland cliff. Here a pitchstone vein, rather more 

 than a yard thick, separates it fi'om the sandstone beneath. At the 

 base of the cliff both are lost for a time under vegetation ; but when 

 this ceases, we find the pitchstone still beneath the felsite. The 

 latter shows a rather platy structure for about three or four feet, top 

 and bottom, most distinct at the latter, being irregularly jointed else- 

 where. Grenerally the felsite is much decomposed, of a pale cream 

 colour, with ferruginous brown stainings, in appearance rather like 

 a soft fine sandstone. The base, however, above the pitchstone, as 

 will presently be described, is much indurated. The pitchstone now 

 thins away, and some irregular outcrops of a greenisli spherulitic 

 felsite make their appearance. These occur chiefly in two little 

 bays in the sandstone about four yards apart. The one further from 

 1 Geol. Mag. Dec. I. Vol. IX. p. 541. 



