EL/E0L1TE-SYEN1TE. \jg 



along which highly double-refracting decomposition-products have 

 in some cases been formed; but otherwise they are perfectly clear, 

 colourless and remarkably fresh. The fracture cracks are apparently 

 determined by stresses on the rock, for they often extend across 

 adjoining felspar-crystals. The ela^olite crystals have been identified 

 by their low double refraction, and in thick sections by the negative 

 character of the uniaxial figure. They are easily decomposed by hy- 

 drochloric acid with the formation of a siliceous jelly which absorbs 

 carmine staining. On evaporation the soluble portion yields cubes 

 of the alkaline chlorides (for chemical composition, see p. 12). 



The Jelspar is light-grey in colour, and in thin section shows 

 the peculiar " moonstone " effect due to a cryptoperthitic structure. 

 Some of it is a definite plagioclase showing lamellar twinning, but 

 by far the larger quantity of felspar in the prevalent grey type of 

 this syenite is orthoclase or anorthoclase with a microperthitic or 

 cryptoperthitic structure. Microcline is unusual, but its structure 

 is often badly imitated by presumably an intergrowth, which between 

 crossed Nicols gives the felspars the structure of a stretched and dis- 

 torted mesh. The threads of the mesh present a red of the 

 first order, whilst the interspaces polarize with yellow of the same 

 order. Sometimes these two constituents of the crystal extinguish 

 almost simultaneously, and from this they may vary up to 9 between 

 their positions of extinction. The crystals occasionally show slight 

 kaolinization. 



The biotite is noticeable for the almost complete absorption of 

 the rays vibrating parallel to the cleavage, and for the olive-green 

 to yellow-green colour of the rays vibrating at right angles to this 

 direction. In convergent polarized light the flakes give the usual 

 figure, indicating a very narrow optic axial angle and negative 

 character of double refraction. 



The iron-ores show their crystal outlines in section, and their 

 clean edges, as well as shapes, serve to distinguish them from the 

 opaque graphite on the same section. The granules separated dum 



( ■• ) 



