508 Iiev. T. G. Bonney — Pitclistones and Fehites of Arran. 
into mounds. I have a slab from the very base, about 1^ incb thick, 
which shows this structure on botb surfaces ; the lower being the 
coarser ; the upper in its fine parallel arrangement almost resembling 
a piece of wood beneath the hark. 
The microscopie structure has been so minutely described by Mr. 
Allport that I cannot do better than quote his words. 1 “ 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 irregulär nucleus, throwing off rays 
towards the circumference ; frequently, however, the centre is free 
from them, and there is then no appearauce of any sort of structure. 
These green patches also occur in the matrix, and both spheres and 
matrix appear to be composed of pi'ecisely 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, 2 and 
that some of the spheres are also composed 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, composed 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 boundary 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 liehen 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 mieroscope, 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. 
2 Exactly the same structure is seen in the felsite from near Drumadoon, mentioued 
above. 
