292 prop. t. g. BONNEr on the [May 1900, 



by the groundmass, and one is the centre of a spherulite. The 

 groundmass is speckled with opacite, is imperfectly perlitic, and is 

 devitrified, being a mass of very minute crystalline granules. The 

 rock differs from the devitrified pitchstones of the Wrekin district, 

 and I have never seen one like it in Britain. (See PI. XX, fig. 1.) 

 Five varieties, containing more or less tourmaline, were described 

 in 1883 1 ; others have been subsequently obtained, two of them 

 showing a fluxional structure, accentuated by the presence of darker 

 streaks. In one, small and rather decomposed, the tourmaline has 

 been determined by microscopical examination of the powder. A 

 slice has been cut from the other (Style Cop), a flattish pebble, rather 

 subangular, of a pinkish-grey colour, and streaked like the last- 

 named, though not quite so frequently. The parts between the 

 streaks seem to contain some minute microliths of tourmaline sparsely 

 scattered, and exhibit a delicate fluxion-structure (devitrified). 

 The dark streaks are formed of divergent bunches of rather small 

 needles of this mineral (the indigo-coloured variety) ; see PI. XX, 

 fig. 3. Here and there, in an ' eye,' we find a few granules of a 

 brownish mineral, with some resemblance, except in shape, to rutile, 

 and perhaps more to cassiterite. 



Another specimen (Style Cop) with a microcrystalline matrix 

 contains a large quantity of tourmaline, granular and acicular, 

 both brown and blue, the former occasionally enclosing the latter. 

 (See PL XX, fig. 2.) In some cases the original mineral probably 

 was felspar, but in others the tourmalines are grouped about or 

 even enclosed between the cleavage-planes of a very brown 

 micaceous mineral. Possibly the rock once contained biotite, 

 which has been converted into tourmaline and a mica allied to 

 lepidolite. 



A third pebble (Baland's Pool Pit) is a reddish felstone with dark 

 spots, almost like ink-stains. This, under the microscope, exhibits 

 a devitrified groundmass, in which tourmaline is locally interspersed 

 with quartz, as though it had replaced a felspathic constituent ; but 

 it also occurs in patches which resemble in outline a rather irregularly- 

 formed biotite. Some similar patches now consist of a more or less 

 tufted chloritic mineral, interspersed with ferrite-grains ; while 

 others of the same external shape are brown tourmaline, sometimes 

 with similar enclosures. In one case part of a grain is distinctly 

 (altered) biotite, while the other is no less distinctly tourmaline ; 

 that is, it exhibits the conversion of a biotite into tourmaline. 

 In some grains of the latter mineral small black needles are arranged 

 parallel with the longer sides, and apparently indicate the position 

 of the cleavage-planes in the original biotite. That may be seen 

 also in the grain mentioned above, just at the parts where the 

 tourmaline has almost replaced the micaceous mineral. (See 

 PI. XX, fig. 4.) These two specimens fully confirm my inter- 

 pretation of the three described in 1883 (he. cit.) to which they are 

 not distantly related. 



1 Geol. Mag. 1883, p. 199. 



