wJm-h have been exposed to High Temperatures. 401 



resemblance to the subject of our description, except that in it the 

 crystals are longer, thinner,* and differently grouped, and a slightly- 

 curved arrangement is perceptible in the ground-mass. 



The second specimen is a large fragment of a slab of black glass, 

 about one-third of an inch thick. In a thin slice this appears, on micro- 

 scopic examination, to be a brown glass, which becomes quite opaque 

 in a narrow zonef near each edge, but in the intermediate part varies 

 from a light-brown to a rather rich umber-brown, each tint being 

 slightly granulated. It exhibits a fluidal structure, the stripes in one 

 part being bent into a fold. The use of high powers brings out 

 specks of opacite, but fails to resolve the glass, so that obviously the 

 colouring matter is very finely distributed. 



A third slide, cut from a specimen given to me by Professor Judd, 

 shows well-defined spherulitesj (deep-brown) in a glass (pale-brown, 

 almost buff). In the centre of some of these spherulites is a cruci- 

 form group consisting of aggregated granules of opacite or ferrite, 

 from which radiate similar but thinner bands, interspersed with 

 clearer fibres, which also have a tendency to group themselves cross- 

 wise, rather than to diverge in all directions. These fibres seem to 

 have a very faint depolarising influence, but they are so small and 

 still so discoloured as to be unfit for examination. These fibrous 

 structures can generally be traced to (he edge of the spherulite, but 

 the dark lines, by which alone it is to be recognised, become so thin 

 that the outer part, with low powers, appears commonly to be struc- 

 tureless. 



A fourth specimen in the museum at University College is labelled 

 " Melted Rowley Basalt, cooled in 13 days ; from upper surface. 

 Eagle Foundry, Birmingham, February, 1836. "|| This is a black glass, 

 with a slightly irregular, sub-conchoidal fracture, rather vesicular 

 in one part, and opposite to this bearing apparently the impression 

 of a mould. Under the microscope, it is a clear glass of a warm- 

 brown colour, by no means dark. It contains a few minute cavities 

 and granules of opacite ; there are some larger vacuoles, circular in 

 section, from O'Ol inch to 0*05 inch in diameter, and a few circular 

 spots, about the same size, slightly grayer and more granular than 

 the rest of the rock ; also one small portion of a deeper brown glass 



* Though occasionally one more " stumpy " may be seen. 



f Mr. Butley observed that an opaque zone occurred in a specimen of vitrified 

 basalt where it joined a fragment of unmelted basalt, which had been dropped into 

 the crucible after this had been removed from the flame. 



J Mr. Rutley observed a rudely spherulitic structure in a slowly cooled specimen 

 of melted basalt (Plate V, fig. 4). 



In the immediate neighbourhood of the central crosses they seem to enclose 

 ferrite granules, arranged along lines with very short cross-pieces. 



|j The date is as printed, but, as each label bears the same signature, " William 

 Hawkes," it may be a clerical error for 1856. 



2 E 2 



