712 
EHYOLITES, Etc. 
]S^o. 1. Eee&tjsson Island, Dattson Stbaits (New G-dinea). 
Sir W. Macgregor’s Collection. Sp. Gr. 2'426. 
Colour, black. Vitreous, containing numerous macroscopic quartz and sanidine 
crystals. The crystals appear to be centres of cracking.* Eracture concboidal. 
Section . — Perfectly isotropic. Glassy base, with plentiful sprinkHug of colour- 
less rods, whose angle of extinction, being parallel with the principal axes of the nicols, 
would indicate monoelinic felspar microliths. Some of the felspars are tabular, 
measuring about 01 inch in length and half or less in width, with very minute inclusions. 
The rods are sometimes plates, looked at edgeways, but this is not always the case ; 
many undoubted rods occur whose arrangement discloses fluxion-structure. The 
quartzes are large, measuring '06 inch to ‘1 inch in length, and filled with gas pores, all 
fixed. The Drawing on Plate 66, fig. 1, exhibits one lenticular inclusion in the left- 
hand corner of the quartz crystal, containing upwards of forty fixed gas pores. The 
drawing also shows the two habits of the felspars, tabular and prismatic. The cracks 
traverse the glassy base and the quartz crystal. Magnetite .sparse, in well-formed 
octahedra. Hornblende (?) green ; in three instances with enclosed crystals of magnetite ; 
a few pale-green hornblende (?) microliths visible under the i inch. The felspar laths 
are mostly stepped, but the tabular felspars are perfectly regular. The higher powers 
also show the existence of deposits along the cracks of the quartzes, ramifying, and spread 
like the dentritic manganese deposits found in the joints of rocks. Very often these 
cracks pass over the margin of the quartz crystal, penetrating into the contiguous glassy 
base. Amongst the quartzes are many glass enclosures, whose contours are invariably 
rounded, but never quite spherical, and the fixed gas bubble is generally at one end. 
Ho. 16. Clonctteet (Queensland). ■ 
E. L. Jack’s Collection. Sp. Gr. 2'297. 
Colour, black. Vitreous. Opaque. 
Section . — A perfect glass, a little traversed by cracks, in no particular direction. 
A slab ’01 inch thick is faintly illuminated between crossed nicols. 
Ho. 3. Mitchell Eitee (Queensland). 
E. L. Jack’s Collection. Sp. Gr. 2'72. 
The rock is of a bluish-gray colour, in spheroidal lumps with conchoidal fracture. 
Section . — Light coffee-coloured glass, perfectly isotropic. Spherical bodies occur 
in the glass, one of which is shown in Plate 63, fig. 1. As there shown, some of those 
bodies are certainly crystalline, and the crystals seem to have gathered round one point 
as a centre, and gi-own outwards, absorbing the material for their growth from the glass. 
The microlites near the centre seem to have had the best chance, and to have availed 
themselves of it, for they are more distinctly crystalline than those more remote from 
the centre, while the groat opacity, and want, apparently, of crystalline structure in the 
periphery, woul d suggest the rejection of the material that was of no use to the growth of the 
crystals. The clear marginal zone round the globular bodies in every case proves that they 
grew by the abstraction of certain substances from the glassy base. {Vide Plate 63, fig. 
l.f) This is almost analogous to the assimilation of food-stuffs by plants and animals. 
* These cracks must be produced by the unequal contraction, on cooling, of the quartz and the glass, 
respectively. This suggests the question, what are the coefficients of expansion for quartz and glass ? 
t The zone may be due to the heat developed in the crystallization, keeping the neighbouring glass 
fluid, while the more remote was getting viscous. Thus no more supplies could be got for the growth of the 
young crystals, except from the envelope immediately surrounding the sphere, which, being leas viscous, was 
more thoroughly drained of the material necessary for the crystal’s growth. 
