100 



Messrs. D. Herman and F. Rutley. 



roughly broken piece of this specimen magnified four diameters is 

 shown in fig. 5, Plate 2. 



Specimen No. 105 is a piece of plate-glass 12 mm. in thickness, 

 having the uneven surface usual in plate-glass before it has undergone 

 the process of grinding. Devitrification in this case has given to the 

 glass the appearance technically known as " burnt," and it was brought 

 about in the ordinary process of annealing owing to the kiln being too 

 hot. The glass was in the stiff, pasty condition suitable for rolling 

 when introduced into the kiln, and was kept at a bright red by flame 

 playing almost directly upon it for about half an hour, during which, 

 and possibly during a short period of subsequent cooling, the devitri- 

 fication was effected. The devitrification of this specimen is quite 

 incipient, and affects merely the two parallel surfaces, one of which 

 is uneven and scratched owing to contact when in a soft condition 

 with the rough bed of the kiln. This latter surface is extremely 

 interesting, as it shows a reticulating series of irregular cracks, 

 traversed in places by straight belts of spherules which are apparently 

 in no way connected with the cracks, and begin and end abruptly in 

 a seemingly capricious manner, fig. 4, Plate 2.* The cracks are 

 similar to those produced in glass by heating it and plunging it in 

 water. Other isolated and larger spherules are also to be seen upon 

 both surfaces of this specimen. Fig. 3, Plate 2, shows one of these 

 surfaces — the upper, as seen by ordinary transmitted light, and 

 magnified 32 linear. The shaded spots represent incipient spherules 

 which fail to show any depolarisation ; the darker spots are decided 

 spherules with strong depolarising power. The unshaded portion of 

 the section also transmits light between crossed Nicols, and is therefore 

 in a state of strain. Under a magnifying power of 1150 diameters 

 the incipient spherules can merely be resolved into brownish granular 

 patches, sometimes approximately round, not uncommonly dumbbell- 

 shaped, or like two coalescing spheres, but usually they are of irregular 

 form, and their general aspect is nebulous. 



Specimen L. Apiece of pale greenish sheet-glass transferred, when 

 in the semi-fluid state suitable for working, to a small pot in which it 

 was maintained during four or five hours at a temperature barely 

 sufficient to permit of its being " gathered." It is traversed by 

 rudely parallel, irregular, flocculent, milky bands. Under a power of 

 about 250 diameters numbers of minute crystallites are visible ; they 

 show no double refraction. Some are stellate, others fusiform or 

 acicular. The latter are often wholly or partially surrounded by fine 

 dusty segregations, which frequently seem to be diminutive divergent 

 spicules. The most common forms have the aspect of monoclinic or 



* This specimen closely resembles some of the spherulitic obsidians of Montana, 

 U.S. Compare this figure with fig. 5, Plate XX, "Quart. Journ. G-eol. Soc," 

 vol. xxxrii. 



