﻿jRev. T. G. Bonney — Pitchdones and FeUites of Arran. 509 



felsite, shows an irregular banded structure ; wavy, mossy, brown 

 bands and elongated streaks of brown dust, giving it somewhat the 

 appearance of a slice of agate ; the latter are full of tolerably clear 

 spherules, sometimes isolated, sometimes crowded ; with crossed 

 prisms these show very clearly radial polarization. In parts of the 

 slide, the dendritic structure described above may be seen ; it is 

 more brightly coloured than in the last; in fact one constituent 

 (quartz ?) in this slide seems to be more distinctly crystalline and 

 the other more earthy than in that specimen. 



With regard then to this particular rock, the spherulitic felsite, the 

 evidence seems to show that it is more closely related to the felsite 

 than to the pitchstone,' but that it is not merely a portion of the 

 felsite altered in situ. It appears to have distinctly flowed at a period 

 subsequent to the intrusion of the ordinary felsite. In this respect 

 its relation seems closer to the pitch stone. It is evidently a rock of 

 extremely local occurrence. I conclude therefore that the pitch- 

 stone, which has everywhere very highly altered the felsite in con- 

 tact with it, has in one or two places actually melted it down, so 

 that the liquefied mass flowed for a short distance before solidifying 

 again. The regularity of the spherules shows that they did not 

 begin to form till the mass was wholly at rest. 



Reviewing then the whole of the evidence, it would appear as if 

 spherulitic 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 which would actually melt it. 



The 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 the accidental presence of some 

 foreign body at the central point. The above examples suggest a more 

 definite explanation, which may at any rate hold good in numerous 

 cases. That spherulites are formed at all may be due primarily to 

 very slight differences either in mineral composition in that part of 

 the glass,^ or in the rate or mode of cooling. What we have noticed 

 seems to show that only very rarely had they any connexion with 

 included substances, and suggests that 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 the 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 about 

 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.) CO3, 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 them associated with the brown glass of pitchstones. 



