90 PEOCEEDI]SGS OE THE GEOLOGICAL SOCIETY. 



as ordinarily sold, if its sides were compressed by its being placed 

 in a hexagonal box. 



Both of the above crystalline aggregates appear to be sharply 

 separated from the enclosing glass, which, I may remark, is often 

 cracked for some little distance round them, as though by strain in 

 subsequent cooling. Although the latter microliths appear to form 

 tufted and sometimes spherulitic masses, like the foriner, by the 

 crowding of the branch-like forms, so that they are compressed 

 together like the twigs in a broom, jet I am disposed to regard the 

 two as distinct varieties, if not distinct minerals. Probably, how- 

 ever, the difference in chemical composition is but slight. Their 

 occurrence, often in the middle of a mass of perfectly homogeneous 

 glass, leads one to suspect that they differ but little from it in com- 

 position, and are crystals of some lime-soda silicate, chemically as 

 nearly as possible identical with the glass*. So far as I can at 

 present ascertain, each is a monoclinic mineral. 



Mr. P. Siemens sent me a specimen of " granulite glass " containing 

 spherulites, taken " out of an old tank-furnace cooled down slowly 

 with about 1000 cwt. of glass in it." The fragment is of irregular 

 form, about 5"x3''x2", of a rich resin-brown glass, containing 

 several spherulites beautifully developed, most of them about 1" 

 diameter, sometimes a little more ; they exhibit a radial structure, 

 with one distinctly zonal in the exterior part, forming a kind of 

 " rind " about '2" thick. The inner part is of a pale yellowish-grey 

 colour and has a slightly unctuous lustre ; the outer has a pinker 

 tinge and deader lustre ; but the outermost zone, perhaps J^-th inch 

 thick, more nearly resembles the interior. When examined under 

 the microscope these spherulites are not very translucent, of a 

 dusty grey colour and rather earthy aspect ; the radial structure is 

 rather irregular, the spherulite being apparently composed of a 

 matted mass of rather curved acicular microliths ; the zones are 

 indicated by darker bands and there are some interesting minor 

 peculiarities on which I must not dwell, except to say that beyond 

 the spherulite is a very thin fringe of minute colourless crystallites. 

 This mineral is probably an aluminous silicate, and resembles that 

 in some spherulites which I have seen in obsidians and pitchstones ; 

 perhaps it is an impure microlithic oligoclase. 



In all these specimens, described above, the various crystallites 

 have formed during the cooling of the mass from a molten condition, 



* Mr. Claudet tells me that some years since, he analyzed a glass and one of 

 the enclosures, and found no substantial difference between them. I may add 

 that Vom Eath's analyses of a pitchstone and a spherulite from Antisana 

 (Andes) give but little difference. Dumas's analyses of a glass and its crystal- 

 lized inclusion, however, show a decided difference : — 



Vitreous part. Crystallized part. 



SiO^ 64-70 68-20 



AIA 3-50 4 90 



CaO 12-00 12-00 



l^a-P 19-80 14-90 



100-00 100-00 



