378 Transactions. — Geology, 



Iron-oxides (magnetite, and sometimes haematite) are 

 frequent in small specks without crystalline boundaries. 

 Hgematite, with a little limonite, is particularly abundant in 

 cracks, and in all probability results from the further oxidation 

 and hydration of the magnetite. 



Beneath the highest power of the microscope (700 dia- 

 meters) no isotropic matter indicating the presence of inter- 

 stitial glass can be seen, but occasionally, especially from the 

 vesicular upper surface of the lava-flow, there are inclusions of 

 a greenish glass in globules of a spheroidal form, the structure 

 in one or two cases being almost pisolitic. 



A few of the sections were examined in convergent polarised 

 light, and the following results were obtained : — 



The large crystal in PI. XLVII., fig. v., shows two hyper- 

 bolic brushes which meet almost in the centre of the field, 

 and the section must therefore be cut almost exactly at right- 

 angles to an axis of elasticity. Since in such a thin section 

 both hyperbolas appear, and when furthest apart they barely 

 disappear from the field, the axis of elasticity must be the 

 acute bisectrix. With the quartz wedge the hyperbolas are far 

 more widely separated, and the result therefore is equivalent 

 to the thinning of the section, wdiich shows that the optical 

 signs of quartz and sanidine are opposite, and since quartz 

 is positive the acute bisectrix in sanidine is negative. 

 Further, since the section was shown to be cut in the ortho- 

 pinacoid -basal zone, the acute bisectrix must be at right- 

 angles to the orthodiagonal. 



The acute bisectrix is thus shown to be negative, and at 

 right-angles to the orthodiagonal, a result in accordance with 

 the general optical properties of sanidine or orthoclase (Dana's 

 " Text-book of Mineralogy," p. 325). 



Plate XLVII., fig. vi., in convergent polarised light only 

 shows one hyperbola, and is therefore not cut at right-angles 

 to an axis of elasticity, although it is cut perpendicularly to 

 the plane of the optic axes. 



1:1 convergent polarised light tridymite only shows faint 

 indications of hyperbolic brushes, which appear only in the 

 edges of the field. 



Plate XLVIIL, fig. ix., the supposed Baveno twin, gives 

 uniform phenomena all over the crystal as far as can be seen, 

 but, as it is not cut at right-angles to an axis of elasticity, this 

 is not certain, as the brushes merely sweep across the section 

 apparently in the same direction in all parts of the crystal. 

 This militates considerably against the supposition that the 

 crystal is a Baveno twin. 



Chemically, this rock presents rather peculiar features, 

 as will be seen from the following analysis. The first 

 two analyses are of the same specimen, while the other 



