Hutton. — On a Leucophyre/row the Selwyn Gorge. 273 



chloritic particles occur, which are very faintly, if at all, 

 pleochroic. 



With a quarter-inch objective the crystallized mineral looks 

 much like olivine, as it is irregularly cracked and has a roughish 

 surface ; but the grains are sometimes cleaved in one direction 

 which is oblique to the positions of extinction ; the surface is not 

 so rough as that of olivine, and they show no trace of decom- 

 position on the cracks. Probably, therefore, they are augite, 

 although they show no twinning, and this supposition is con- 

 firmed by the chemical analysis. These crystalline grains con- 

 tain numerous minute irregularly shaped, but generally angular, 

 bright particles, generally of a greenish hue ; and sometimes 

 short dark lines, curved or straight. Minute liquid-cavities, 

 occasionally with a bubble, can be found, but are rare. 



The ground-mass is a finely granular crypto-crystalline mass, 

 of low polarization colours, colourless or very pale greenish, and 

 consists of a water-clear colourless glass with numerous short 

 rods and minute irregularly- shaped particles. In fact, it is 

 saussurite. 



The chemical composition of the rock is as follows : — 



Percentage Composition. 



Hygroscopic moisture ... ... -30 



Loss on ignition .. . ... ... 2-81 



Silicic anhydride ... ... 47*41 



Aluminic acid ... ... ... 12*66 



Ferric oxide ... ... ... 10*88 



Ferrous oxide ... ... ... 2*52 



Manganous oxide ... ... *73 



Calcic oxide ... ... ... 11*21 



Magnesic oxide ... ... ... 7*42 



Potassic oxide ... ... *22 



Sodic oxide ... ... ... 2*92 



Carbonic anhydride ... ... *26 



Loss, and undetermined ... ... *66 



100*00 

 Specific gravity 3*048. 



This analysis shows that there is little or no olivine in the 

 rock, but that it is composed roughly of about two-thirds of a 

 lime-magnesia augite* and one-third labradorite with some iron 

 ore ; while microscopical examination shows that the iron ore 

 is ihnenite, and that the labradorite has been converted into 

 saussurite. The rock, then, consists of a pale-green augite 

 (diopside ?) in a more or less abundant base of saussurite, 



* Of approximately the same composition as that from the Whin Sill, 

 described by Mr. Teall in " Quar. Jour. Geol. Soc," vol. xl., p. 648, but with 

 less iron. 



18 



