506 Iiev. T. G. Bonner/ — Pitchstones and Felsites of Arran. 
described by Mr. S. Allport, 1 namely, a felsitic structure outside the 
circle, and a fibrous radial structure inside, the division between 
them not being quite so sharp or regulär 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. — Splierulitic Felsite. Diagrammatic sketch from microscope. The radial 
structure in the spherulites 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 (fenuginous ?) dust. 
The next rock to be examined is the spherulitic felsite on the 
Corriegills shore. It has been noticed by Maccullocli 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 from 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. Generally 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 irregulär outcrops of a greenish 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. Yol. IX. p. 541. 
