Vol. 62.] CATAEACTS OF THE EIVER MADEIEA, ETC. 109 



grained syenite with a brownish tint. Under the microscope the 

 rock (M12; specific gravity = 2*69) seems to be mainly composed of 

 felspars ranging up to a millimetre in diameter, the commonest being 

 an acid oligoclase with very fine twin-lamellation. Microcline and 

 microperthite are also present, but there is very little quartz. 

 There are traces of a pleochroic brown augite, now more or less 

 decomposed. A green hornblende of later formation, with a 

 maximum extinction of about 24°, appears to have been partly 

 formed from the decomposition of the augite. Sphene, haematite, 

 magnetite, chlorite, and epidote are also present, in most cases as 

 alteration-products. 



The syenite is intersected by a vein of coarsely-crystalline rock 

 mainly composed of dull-red felspar, chiefly microperthite. There 

 is a considerable quantity of quartz, interstitial or in large grains, 

 occasionally occurring as rounded or idiomorphic inclusions in 

 the felspar, and a little dark-green hornblende which is much 

 decomposed. A number of well-developed zircons are also present. 



There are also veins (M 11, PL V, fig. 4; specific gravity = 2'79 to 

 2*84) with a groundmass that is aphanitic to the naked eye, and small 

 porphyritic crystals of felspar up to about a millimetre in diameter. 

 On microscopical examination these prove to be an acid labradorite. 

 Lath-like in section, they appear to form plates developed parallel to 

 the brachypinakoid, or prisms parallel to the clinodiagonal. They 

 are, to a considerable extent, altered into colourless decomposition- 

 products. These are linear, blade-like, or lenticular in shape, as 

 seen in thin sections, and usually fibrous and feathery in appearance. 

 They often lie obliquely to the length of the felspar-sections. The 

 direction of extinction corresponding to the vibrations with the 

 less velocity is usually parallel with the length or fibrous structure. 

 The birefringence is sometimes as much as 19 thousandths (-019). 

 The refractive index appears to be higher than that of the felspar 

 (say 1*557), or of a mixture of cedar-oil and mono bromo naphtha- 

 lene, with a refractive index of rather less than 1*57. It is, on the 

 other hand, lower than that of the slower (ordinary) vibrations 

 of the mineral allied to calcite, referred to below, or than that of 

 mono bromo naphthalene, 1*66. These determinations were made 

 by the Becke method. In the case of the mixture and the mono- 

 bromo naphthalene, a drop of the liquid was placed on the edge 

 of the uncovered section that was free from Canada-balsam, and 

 the Becke method was applied to the marginal crystals. 



After treatment with hydrochloric acid and washing, the mineral 

 was not stained by a solution of rhodamine, thus showing an 

 absence of silicates yielding gelatinous silica when treated with acids. 

 But, on drying in a hot air-bath, and then moistening with a 

 solution of rhodamine, a stain was produced showing it to be a 

 hydrous silicate. 



From the foregoing characters I believe these alteration-products 

 to consist of a hydrous mica, probably paragonite. 



In a few cases a mineral, which is similar in general appearance, 

 seems to have an oblique extinction, the angle between the length and 



