398 Prof. T. G. Bonney. On Specimens of Rock [Jan. 28, 



changes are obvious at a glance the development of a number of 

 vacuoles,* and the presence of a considerable amount of brown glass. 

 The former are usually spherical, or nearly so, with well-defined, 

 " clean-cut " edges. The smaller of them are often lined, perhaps in 

 some cases partly filled, with opacite, and are thus rendered more or 

 less opaque. In some cases, very tiny clear crystallites are inter- 

 spersed among the blackest dust. The ground-mass of the rock 

 consists of somewhat rounded, or sub-angular, grains of a clear 

 mineral set in a base of light-brown glass. Of these, the majority 

 unquestionably represent the quartz of the original ground-mass, but 

 I think that some are felspar. Most of the mica has also vanished, 

 though here and there a small remnant may be detected. The 

 cavities in these quartz grains appear to me to be less numerous but 

 a little larger than before. Often they are seemingly empty, but in 

 some the bubbles still remain apparently without any change. The 

 larger quartzes of the unaltered rock now appear as grains which 

 have a rather irregular outline, and are much cracked. Here also 

 the cavities seem to be reduced in number, but not to have increased 

 in size. The larger felspars have disappeared; but their position, on 

 closer examination, can be identified by irregular patches of dirty 

 glass, in which are very numerous vacuoles; these generally are 

 rather small in size, and coated with dark dust. In parts of the glass 

 minute fibres may be observed, which, however, do not act on polar- 

 ised light ; in others a tiny patch or spot is feebly doubly-refracting. 

 In other parts (especially, I think, where the vacuoles are Lirger, but 

 less numerous) the glass is a deep-brown, barely translucent, or even 

 opaque. Here and there, in the glass generally, and near the edges 

 of included grains, groups of minute vermicular cavities appear, and 

 a few microliths occur in the slide, which are so small that it would 

 not be safe to offer an opinion as to their nature or age. 



Practically, the effect of the heating has been to melt down the 

 felspathic and the micaceous constituents, without very materially 

 affecting the quartz, and to render the mass vesicular. 



At Les Talbots, in Guernsey, brick is made of a material which is 

 mainly, if not wholly, disintegrated granite. An over-burnt specimen 

 which I have examined is a brown slaggy glass, fall of mineral frag- 

 ments.! These, under the microscope, are seen to be chiefly quartz 

 and felspar. The grains of the former are traversed by numerous 

 cracks, and the pieces appear to have been sometimes displaced, but 

 to be otherwise unaffected. Those of felspar also are cracked. Nume- 

 rous vesicles have formed : the majority very small, but some are easily 



* These, it will be remembered, were developed in the experiments described by 

 Mr. Rutley, both in a pitchstone from Arran and an obsidian from Montana. 



'f The specimen was collected by the Kev. E. Hill, F.GJ-.S. In 1888 he took me 

 to tlie locality. 



