432 



PEOP. J. W. JTJDD ON THE VOLCANIC EOCKS 



fuses, though with but little intumescence, and forms a colourless glass. 

 In separating along the lines of perlitic cracks and around the mi- 

 crolites of the rock, this secondary product forms globiform particles 

 similar to those described in the case of the pitchstone of Meissen by 

 Vogelsang *. (See Plate XIII. fig. 6.) 



The white hydrated glass is so soft that it can be scratched with 

 the thumb-nail. When placed in cold water it rapidly breaks up 

 and falls to powder, the unweathered crystals of felspar separating 

 and remaining intact. Dried at 110° C., the mass loses 12-4 per 

 cent, of its weight, and on ignition 10*1 per cent, more is removed. 

 It thus appears that the white decomposition-product contains no 

 less than 22-5 per cent, of water and other volatile products. The 

 dried specimens of this substance contain nearly the same proportion 

 of silica as the original rock. 



The material which seems to come nearest to this in composition 

 and properties is the " alumo-calcite " of Kersten, found at Eiben- 

 stock, near Rezbanya, which has a hardness of 1 to 2, a specific gra- 

 vity of 2*1 to 2*2, and contains 86-6 per cent, of silica with a certain 

 quantity of alumina, lime, and water. Substances of this class pro- 

 bably constitute an intermediate stage between the acid glasses and 

 the opals which so frequently result from their decomposition. 



The felstones, tuffs, and ashy sandstones referred to by Mr. Durham 

 are usually in such a highly altered condition that in most cases it 

 is not possible to do more than speculate concerning their original 

 constitution and characters. 



EXPLANATION OF PLATE XIII. 



Fig. 1. Pyroxene-andesite (augite-enstatite-andesite) of NorthSeld Quarry, 

 near Newport, Fife, N.B., as seen with a magnifying-power of 36 

 diameters. The base consists of a glass filled with felted microlites 

 of plagioclase felspar (Microlithenfilz). Through this base are scat- 

 tered crystals and aggregates of crystals. One of these clusters is seen 

 near the centre of the field, and consists of clear, almost coloiu*less 

 augite, the crystals being twinned and having rovmded outlines ; the 

 unaltered condition of these augite-crystals is in marked contrast with 

 that of the associated enstatite crystals, which are much decomposed, 

 and almost completely converted into bastite, enough of the original sub- 

 stance remaining, however, to give the characteristic rhombic extinc- 

 tion and other optical properties of enstatite ; between the augite 

 • and the enstatite crystals there is visible a third colourless substance, of a 

 granular character, which is probably the result of an alteration of fel- 

 spar. The whole aggregate gTeatly resembles a fragment broken off from 

 a deeper-seated crystalhne rock, with its edges fused and rounded ; and 

 we seem to have an example of the structure to which I have given 

 the name of " glomero-porphyritic." The magnetite and felspar 

 microlites are crowded in the glass immediately around this cluster 

 so as to form a dark zone about it. 



Fig. 2. Pyroxene-andesite (enstatite-augite-andesite) from Causewayhead 

 Quarry, near Newport, Fife, N.B., as seen magnified 35 diameters. 

 Enstatite (with a little augite), well crystallized in prisms and in 

 rounded granules, is scattered through a base in which lath-shaped 

 felspars have become less distinctly individualized. At many points 



* ' Die KrystaUiten,' p. 145, Taf. xv. figs. 4, 5. 



