It is well known that in the Schroeder Van der Kolk method of comparing the refractive index of a crystal grain with that of the material in which it is mounted, the effect can be reversed by moving the microscope sub-stage condenser, and that the position of this condenser must therefore be carefully controlled if ambiguities in the interpretation of the observations are to be avoided. This fact is mentioned in the majority of text-books and is referred to in a general manner by Wright and by Winchell. No clear explanation of the reversal is offered, however, and students are consequently inclined to forget this important limitation in the method or, alternatively, to forget in which sense the movement of the condenser influences the effect: unless a control experiment is invariably performed errors may therefore result.

If α, β, and γ are the three angles between a line passing through the origin and the three rectangular co-ordinate axes, then the equation connecting them is as follows:

cos2α+cos2β+cos2γ = 1.

This is the law of direction-cosines of a line.

Substituting (cos 2α+1)/2 for cos2α, &c., we get:

cos 2α+cos 2β+cos 2γ = −1.

During a petrological examination of samples of bauxite from Boolarra, in south Gippsland, Victoria, it was noted that some specimens of the bauxite, which is largely derived from Tertiary olivine-basalt, contained numerous grains of yellow-brown to amber-yellow leucoxene. The leucoxene is clearly pseudo-morphous after ilmenite, residual particles of ilmenite being enclosed in many of the leucoxene grains. Most of the leucoxene grains are opaque, but occasional grains are translucent to transparent, though isotropic. Some of them show parallel markings suggestive of cleavage, but probably a residual structure from the replaced ilmenite. In view of the highly aluminous nature of the enclosing rock, there seemed some possibility that this mineral might be the little-known aluminium titanate, xanthitane. It was thought, therefore, that if a pure sample of the mineral could be prepared, a chemical analysis would establish its identity.

The Chapel, or Charlestown Main, Limestone horizon lies in the Lower Limestone Group of the Carboniferous Limestone Series. At Chapel quarry the limestone is underlain by a thick sill of quartz-dolerite. The contact, however, is not exposed but probably lies only a few feet below the quarry floor. Other intrusions are seen in the vicinity: a very thin dike of decomposed basalt, which connects with a small ‘float’ of white trap, cuts the limestone about 30 yards north-eastwards of the point where the quarry crosses the Chapel road, and a small sill of olivine-dolerite or teschenite occurs at a horizon about 100 feet above the top of the limestone. In the quarry the limestone is about 60 feet thick. The contact-alteration described in this paper is most evident near the base of the limestone face and its extension laterally in the quarry has no relation to the outcrop of the thin dike. The alteration is therefore considered to be essentially connected with the underlying, unexposed, sill of quartz-dolerite, and, as will be shown ih the following pages, has been effected by the action of hot solutions rather than by thermal reconstruction.