﻿508 Rev. T. G. Bonney — Pitchstones and Fekltes of Arran. 



into mounds. I have a slab from the very base, about 1\ inch thick, 

 which shows this structure on both surfaces ; the lower being the 

 coarser; the upper in its fine parallel arrangement almost resembling 

 a piece of wood beneath the bark. 



The microscopic structure has been so minutely described by Mr. 

 Allport that I cannot do better than quote his words.' " In a very 

 thin section, in ordinary light, the spheres exhibit a well-defined, 

 circle, bounded by a line of minute grains of iron oxide, but the 

 fibrous structure is not so distinct ; in fact, it then appears to be 

 simply a radial arrangement of the particles of a fine dust scattered, 

 through a dull uniform base ; dark greenish aggregations of this 

 substance sometimes form an irregular nucleus, throwing off rays 

 towards the circumference ; frequently, however, the centre is free 

 from them, and there is then no appearance of any sort of structure. 

 These green patches also occur in the matrix, and both spheres and 

 matrix appear to be composed of precisely the same substance. 

 Placed between crossed nicols the appearance is completely changed, 

 and it is at once seen that the matrix has a felsitic structure,^ and 

 that some of the spheres are also comjDOsed of portions of the same 

 substance, which have, however, undergone a process of aggregation 

 and radial arrangement in globular masses; but the felsitic structure 

 is still quite as evident as in the base. Many of the spheres are, 

 however, coinposed of two or more concentric layers ; in some there 

 is a felsitic nucleus surrounded by radiating groups of green dust ; 

 in others the nucleus consists of grains of quartz only." I may add 

 that where spherulites are in contact, there is a divisional line 

 between them, as in the Lipari and Drumadoon specimens, and some- 

 times the spaces between the polygonal boundai'y lines and the circle 

 show a structure intermediate between the normal felsitic and the 

 true radial ; also that this spherulitic felsite exhibits great variety of 

 structure ; a piece taken at the distance of a yard from one of 

 the parts where these spherulites are best developed showing com- 

 paratively few of them, and these very small, often imperfect, and 

 with a linear arrangement. A slide from this last specimen shows 

 that the felsitic structure mentioned by Mr. Allport prevails in most 

 parts ; and the spherulites are isolated and not very well developed ; 

 a slight banded structure is apparent in the rock ; and here and there 

 also a curious structure something like a lichen or dendritic growth. 



A slice cut vertically from the slab at the base of the felsite in the 

 cliff shows that this structure prevails throughout it. With crossed 

 prisms the effect is really pretty, a peculiar arborescent growth, like 

 some lichens or sea- weeds, standing out with a sort of stereoscopic 

 effect in various tints of milky grey against a dark ground. Here 

 and there is a trace of spherulitic structure, and the bands, which 

 are not nearly so distinct under the microscope, have rather a 

 streaky aspect. A slice from the base of the felsite on the shore, 

 where it approaches most nearly in appearance to the spherulitic 



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



'^ Exactly the same structui'e is seen iu the felsite from near Drumadoon, mentioned 

 above. 



