A^fJS'IVEESAKT ADDRESS OF THE PRESIDENT. 89 



with which probably the silica and alumina percentages would more 

 nearly correspond. Nevertheless, though this fact obliges me to 

 hesitate at present as to the identification of the minerals formed 

 during devitrification, these specimens have proved of great assis- 

 tance*. 



It will be convenient to describe very briefly the results of my 

 examination of the various glass specimens referred to above, before 

 indicating their bearing on the devitrification-structures of natural 

 rocks. 



In ordinary window-glass two types of crystals appear to be 

 developed t. One consists of long acicular prisms of a clear mineral 

 with a satiny lustre, like that of pectolite. These form spherulites 

 (sometimes more than an inch in diameter in the body of the glass), 

 but they also, as we might expect, form a more or less mamillated 

 layer, starting from the exterior portion of the mass, especially 

 where in contact with the sides of the pot. Both are composed of 

 thickly crowded tufts of acicular or hair-like microliths, exactly like 

 the growth of micro crystalline quartz which we can study in the 

 chalcedonic linings of cavities in rocks. Sometimes they form 

 radiate hemispheres, the plane face being the surface of the glass, 

 the centre not seldom a httle speck, which appears to have been one 

 of the clay granules mentioned above. The second type is a mineral 

 of a less satiny lustre and apparently not quite without colour, the 

 larger aggregates having a faint ochreous-grey tint. Its habit of 

 crystallization appears to be entirely different. Commencing with an 

 elongated flattish prism, smaller prisms attach themselves to the 

 sides at angles of about 60°, and to these in like way others are 

 added. Occasionally the rachis, as I may term it, of the leaf 

 becomes curved. These, again, combine into stellate six-rayed 

 forms, reminding us of the well-known snow crystals, and develope 

 into flattish hexagons like those figured by Vogelsang in the eighth 

 and ninth plates of his work 'Die Crystalliten,' except that the outer 

 boundary appears sharper than in those on the latter plate. A 

 remarkable twinning now takes place, two composite crystals 

 placing themselves at angles of 60° with the plane of the first plate, 

 so that a vertical section would give us a six-rayed star. This 

 process continues, but with a certain dominance of crystals lying in 

 the original direction, so that its result is an aggregated mass of 

 somewhat flattened forms, the longer diameter of which sometimes 

 measures nearly an inch. At the ends it is rather concave, and its 

 sides still retain a somewhat hexagonal shape. I may give a general 

 idea of the appearance by comparing one of these to a ball of cotton 



* I am, of course,, aware of the glass formed from melting basalt ; but on 

 account of the rai-itj of glasses and structures suggestive of "devitrification" 

 among the more basic rocks, I have not thought it worth while to spend much 

 time over them. 



t Mr. Claudet informs me that these spherulites were produced by stopping 

 all the orifices of a furnace, and allowing it to become cool very slowly, the 

 time occupied before the cooling was completed and the pots were withdrawn 

 being from 8 to 10 days. 



