222 Grenville A. J. Cole — Igneous Rocks of Stanner. 



brought out distinctly, but, in place of the formation of a black 

 cross, extinction occurs in two, three, or more sectors at a time, 

 which yield coloured polarization in intermediate positions. This 

 points to secondary crystallization having set in, with the usual 

 passage from a glassy or cryptocrystalline to a microgranular con- 

 dition. Just as the most detailed structures of the original glass, 

 down to the forms of the minute included crystallites, are preserved 

 in the secondary granules of our ancient rhyolites, so here the 

 spherulitic form and structure remain, in spite of more complete 

 crystallization of the mass. Under higher powers the polarization- 

 effects of the sectors are themselves seen to be complex. A delicate 

 feathery structure has developed, sometimes in pinnate diverging 

 lines, sometimes as a mere network, and the areas into which the 

 spherulite has divided have often become thus minutely pegmatitic. 

 The network commonly extinguishes in one position, while the 

 interspaces extinguish in another — a point best observed when 

 magnified some 400 diameters. 



MM. Fouque and Levy have figured 1 a quartz-porphyry from 

 near Saulieu, Cote-d'Or, which is almost the precise counterpart of 

 the felsitic rocks of Stanner. They find in the spherulites sur- 

 rounding porphyritic and corroded crystals of quartz a network 

 that extinguishes simultaneously with the pre-existing central 

 crystal ; a complete predominance of this material in the spherulite 

 gives rise to what they have termed Quartz globulaire. The micro- 

 pegmatitic structure of these authors appears to give similar effects, 

 as if the radiating lines of quartz were an attempt to enlarge in 

 optical continuity the crystal about which they have developed. 2 A 

 dyke running E.N.E. in the wood upon the northern slope of Stanner 

 contains, in addition to a multitude of orthoclase and plagioclase 

 microlites, numerous corroded grains of quartz ; each of the latter 

 has served as a centre of devitrification (Fig. 2), and in several 

 instances the highly-interesting observation of MM. Fouque and 

 Levy may be repeated on these British spherulites. When com- 

 parison is made with similar rocks of much more recent date, it 

 appears at least probable that the mesh-work structure (of quartz 

 and felspar?) is in reality of secondary origin. So far as I am 

 aware, it does not occur, at any rate freely, among the compactly- 

 spherulitic rhyolites of later Tertiary days ; but in a dyke traversing 

 the Lias of Broadford, Isle of Skye, and belonging to the pre- 

 Miocene series of eruptions, we have what looks remarkably like 

 the commencement of such a structure by alteration. The spheru- 

 lites mostly exhibit a dark cross, with, however, a tendency towards 

 a granular condition ; and the meshy micropegmatitic appearance 

 occurs only in a few individuals, in a manner strongly suggesting 

 a development of quartz among their previously homogeneous fibres. 3 

 It is quite conceivable that such secondary quartz might adopt the 

 orientation of an included and rounded crystal, just as it is known 

 to do when deposited in sandstones from solution. 



1 Mineralogie micrographiqne, pi. xii. fig. i. a Ibid, pp. 193-94. 



3 Cf. Judd, Quart. Journ. Geol. Soc. vol. xlii. p. 72. 



