105 



was only by a chemical examination that its true character was 

 ascertained. The mineral was found only at the locality No. 5, 

 and in small quantity. The two types of zircon, thus, occur 

 associated. However, they apparently belong to two different 

 generalions. The crystals of the second type belong to a far 

 later epoch of formation than those of the first type. They 

 generally occur implanted on aegirine and are surrounded by 

 the same mineral in the form of green translucent needles, 

 which are of secondary origin. Associated with them occur 

 further the minerals ancylite and yttria-apatite, both of which 

 belong to a comparatively late generation. 



The optical properties of the two crystal types are quite 

 the same. Sections orientated perpendicular to the vertical axis 

 show very distinct cleavages parallel to faces of the prism of 

 the first order. Parallel to the same directions the mineral is 

 also divided into differently coloured zones. Sometimes there is 

 a large square central field, which is almost colourless and sur- 

 rounded by a broader marginal zone, which has a tolerably 

 intense yellowish brown colour. Sometimes the central field is 

 yellowish brown and the marginal zone colourless. The colour- 

 less portions of the sections often contain numerous grey in- 

 clusions thickly scattered through the substance, which do not, 

 even when highly magnified, show any regular outlines. Not 

 infrequenlly they are zonally arranged parallel to the aforesaid 

 zones. Generally there are several zones of coloured and of 

 colourless substance from the centre to the outline of the sec- 

 tion. The outermost zone is always uncoloured, but very narrow. 

 Sometimes there is also a zonal structure parallel to the tetragonal 

 prism of the second order (a=^{lOO}). This structure is often 

 perceptible only under high magnifying powers in the outer parts of 

 the section. When a coloured zone, e.g. parallel to (110), borders 

 upon a colourless one, e.g. parallel to (llO), the line between 

 them (parallel to (010)) is very sharp. The double refraction is 

 positive and the same in the differently coloured zones. 



