MR J. HORNE AND MR J. J. H. TEALL ON BOROLANITE. 175 



Water and carbonic acid are present, but were not determined. The reaction of this 

 substance with acid is suggestive of cancrinite, but the occurrence of sulphuric acid 

 points to the conclusion that it is an alteration product after a mineral of the sodalite 

 group. 



Wollastonite was found only as a constituent of certain inclusions in a specimen col- 

 lected on the south side of Sgonnan Mdr. These inclusions are of a greenish white colour. 

 When examined with a lens they are seen to consist mainly of a colourless mineral 

 having a pearly lustre and a fibrous structure. This mineral is decomposed by hot 

 hydrochloric acid, and the solution, after the addition of dilute sulphuric acid, yields 

 gypsum crystals in abundance. Its specific gravity is 2*895. By detaching a small 

 fragment and crushing it upon a slide, numerous long fiat laths are obtained. These 

 invariably give straight extinction. When examined in convergent polarised light they 

 fall into two groups : — (a) Those which show the emergence of an optic axis near one 

 margin of the field of view and that of a bisectrix on the opposite margin ; (6) those 

 which show the emergence of an optic axis nearly in the centre of the field of view. 

 Observations on flakes of the first group prove that the optic axial plane is at right angles 

 to the length of the flakes, and that the minor axis of depol'arisation is parallel to the 

 length. We may, therefore, infer that the acute bisectrix is the least axis of elasticity 

 and that the double-refraction is positive. All these facts point to the conclusion that 

 the mineral is wollastonite. The flakes above referred to are determined by the two 

 dominant cleavages. The straight extinction is a consequence of the fact that the edge 

 formed by the meeting of the two principal cleavage planes is at right angles to the plane 

 of symmetry of the monoclinic mineral, and coincident, therefore, with the mean axis of 

 elasticity.* 



The greenish tinge of the aggregates of wollastonite is due to the presence of a large 

 number of extremely minute granules of green pyroxene. It is interesting to note that 

 the aggregates of wollastonite from Willsburg, N.Y., U.S.A., also contain grains of a 

 similar pyroxene. 



3. One of the most striking features of these remarkable rocks is the pseudo-porphyritic 

 aspect due to the white or more rarely pink patches. Under the microscope these 

 patches are seen to be in all cases aggregates. Orthoclase in the form of allotriomorphic 

 grains is the principal constituent, but there is generally more or less of the substance or 

 substances which gelatinise with hydrochloric acid and possess other characters indicating 

 the former presence of nepheline and sodalite. Micro-pegmatitic intergrowths of felspar 

 and the indefinite substance are not uncommon. We are indebted to Professor Derby 

 of Sao Paulo for an interesting suggestion as to the nature of these patches. A 

 specimen of the rock containing the white patches was given to him, and in writing to 



* Particulars as to the means by which wollastonite was identified are given because they do not appear to be 

 generally employed by petrologists. It is often much easier to identify a mineral by studying the form and optical 

 characters of the small fragments obtained by crushing than by examining thin sections. A description of the 

 ordinary rock -forming minerals from this point of view would be of great service, and anyone who will undertake 

 the work will confer a benefit on petrologists. . . 



