﻿234 



DR. A. HOLMES ON THE TERTIARY 



[vol. lxxii, 



prismatic habit, elongated parallel to the a axis. The three 

 refractive indices are respectively slightly above, equal to, and 

 slightly below 1/526 (monochlor benzol) according to the direction. 

 Carlsbad twinning is common, but the fine striatums due to albite 

 twinning often seen in anorthoclase have not been detected. The 

 extinction, however, is mottled and undulose. Small inclusions are 

 present, frequently with a peripheral arrangement. Most of them 

 are minute prisms of segirine, but a few highly refractive blebs, 

 which exhibit high polarization-colours, are probably zircon. 



Cossyrite occurs in ragged patches, and in thin wedges 

 occupying the angles between adjacent felspars. Except in 

 especially thin sections and near the edges, when the deep-red and 

 brown pleochroism becomes visible, the mineral is practically 

 opaque. Small grains of katoforite and arfvedsonite are 

 associated with the more absorbent amphibole. 



^Egirine and segirine-augite lie among the felspars in 

 shreds and granular aggregates. The pleochroic scheme of the 

 segirine is as follows : — 



x (nearly = c) blue-green. 



y = b yellow-green. 



z (nearly = «) ... yellow-brown. 



Magnetite, probably titaniferous, is present in masses of about 

 the same size as those of cossyrite. It can readily be distinguished 

 from the latter by observation in reflected light. Red ferruginous 

 stains accompany the amphiboles in some cases, and probably 

 represent an excess of ferric oxide over that required for their 

 crystallization. 1 



Chemical Composition. 



Analysis of a jterfectly fresh specimen (153) gave the following 

 results : — 



Molecular Mineral Composition 



Percentages. Proportions. (Norm). 



SiO 61-02 1-017 Quartz 1-741 



Al,6. 3 17-11 -168 Orthoclase 26-69 I Salic 



Fe'O, 4-37 -027 Albite 55-02 f =87-62. 



Feb 3-42 -047 Anorthite 4-1 7 J 



MgO 0-63 -016 



CaO 1-20 -021 Diopside 1-401 



Na,0 6-51 -105 Hypersthene ... 3-28 I Fernic 



K.,0 4-47 -048 Magnetite 6-26 f =12-00. 



H"0 +- ... 0-31 ... Hmenite 1-06 J 



99-62 



H.,0- ... 0-33 



Tib., 0-59 -007 



MnO 0-34 -005 Water 0-64 



Total 100-30 Total 100-26 



Class I Persalane. 



Ra = 2-26 X 10 -12 grms. per grm. Order 5 ... Canadare. 



Specific gravity = 2-58. Rang 1 ... Nordmarkase. 



Subrang 4 Nordmarkose. 



1 For the primary character of hfernatite and limonite in certain igneous 

 rocks, see G. M. Murgoci, Amer. Journ. Sci. ser. 4, vol. xx (1905) p. 145. 



