Rev. T. G. Bonnei y — Pitchstones and Felsites of Arran. 509 
felsite, shovvs an irregulär banded structure ; wavy, mossy, brown 
bands and elongated streaks of brown dust, giving it soraewhat the 
appearance of a slice of agate ; tbe latter are full of tolerably clear 
spherules, sometimes isolated, sometimes crowded ; with crossed 
prisms these show very clearly radial polarization. In parts of tbe 
slide, tbe dendritic structure described above may be seen ; it is 
xnore brightly coloured than in tbe last; in fact one constituent 
(quartz ?) in this slide seems to be more distinctly crystalline and 
tbe other more eartby tban in tbat specimen. 
With regard tben to tbis particular rock, the spberulitic felsite, tbe 
evidence seems to show tbat it is more closely related to tbe felsite 
tban to the pitchstone , 1 but that. it is not merely a portion of tbe 
felsite altered in situ. It appears to have distinctly flowed at a period 
subsequent to tbe intrusion of the ordinary felsite. In tbis respect 
its relation seems closer to tbe pitchstone. It is evidently a rock of 
extremely local occurrence. I conclude tberefore tbat tbe pitch- 
stone, wliich bas everywhere very bigbly altered the felsite in con- 
tact with it, bas in one or two places actually melted it down, so 
tbat the liquefied mass flowed for a short distance before solidifying 
again. Tbe regularity of the spherules shows that they did not 
begin to form tili the mass was wholly at rest. 
Eeviewing tben tbe wbole of the evidence, it would appear as if 
spberulitic structure were generally one of the very last to be set 
up, so as to be even capable of being produced in a rock by a tem- 
perature a little lower than that whicb would actually melt it. 
Tbe above observations also seem to throw some light on the 
cause which produces it. This seems generally to be vaguely stated 
as ‘ concretionary,’ or the result of crystalline forces ; devitrification 
being often supposed to be started by tbe accidental presence of some 
foreign body at the central point. Tbe above examples suggest a more 
definite explanation, whicb may at any rate hold good in numerous 
cases. Tbat spherulites are formed at all may be due primarily to 
very slight diflerences either in mineral composition in that part of 
the glass , 2 or in the rate or mode of cooling. Wbat we bave noticed 
seems to show that only very rarely had they any connexion with 
included substances, and suggests tliat they have rather grown in- 
ward than outward. Suppose then a cooling mass, already in a 
semi-solid condition, to be losing heat very slowly and uniformly in 
all directions, it will thus be thrown into a state of strain from con- 
1 I am indebted to a College friend for tbe following additional evidence. 
The spherulitic felsite agrees in composition more closely with the other felsite 
(allowing for its state) than with the pitchstone; the last having at least 2 per cent. 
less SiOo. Again, when the powder of all three is heated to redness for ahout 
10 m., that of the two felsites does not cohere, and afterwards cannot be distinguished 
in colour— being an ochreous grey; while that of the pitchstone coheres and is 
brownish stone-grey. Again, on fusing both the spherulitic felsite and the pitch- 
stone with (K. Na.) CO 3 , the former assumes a yellowish-green tinge, and the latter 
a rather clear bright green, showing there is more Mn or less Fe in it. The pitch- 
stone also (though the coarser powder) decomposed readily with the fusing mixture, 
while both the felsites left a residue, which required prolonged fusion. 
2 As when we find theiu associated with the brown glass of pitchstones. 
