Vol. 62.] CATAEACTS OF THE EIVEE MADEIEA, ETC. Ill 



The most conspicuous constituents are small felspars in lath-like 

 sections, rather narrower in proportion to the length than in the 

 case of the porphyritic crystals. They are generally less altered, 

 although it is difficult to say why that should be so.* Between 

 them are numerous rodlike crystals, brownish-green in colour, and 

 having usually oblique extinction. The angle between the direction 

 of vibration of the slower wave-front with the length varies from 

 59° to 90°, according to the position of the section. The relative 

 retardation is small, but this is largely due to the fact that the 

 mineral, when lying parallel to the surface of the section, does 

 not occupy the whole thickness. The pleochroism is feeble, the 

 greatest absorption being usually, but not always, in the direction 

 of the length. Approximately-rectangular sections appear to be 

 fairly common, and have extinctions parallel to the diagonals. The 

 mineral is probably a pyroxene. It resembles acmite in external 

 form and in colour, but is allied in its optical characters to aegirine- 

 augite. It appears to be altered in places, partly into opaque 

 ferruginous products, partly into micaceous material similar to 

 that derived from the alteration of the olivine. 



Scattered about through the mass are numerous small crystals 

 and rounded grains of magnetite, and probably ilmenite. The 

 interspaces are filled with a glass-like material, which probably 

 represents an original glass ; it is usually feebly anisotropic in 

 ill-defined bands, and may consist either of analcime or of a secondary 

 glass. In it are numerous colourless needles, which appear to be 

 identical with the larger similar crystals which I have identified as 

 aegirine-augite. Similar needles are also occasionally found in 

 some of the felspars. 



The rock was analysed by Mr. G. S. Blake, with the following 

 result : — 



Percentage Molecular Composition without 



composition, proportions. carbonic acid and water. 



Si0 2 43-88 -7313 48-04 



Ti0 2 0-33 -0041 0-36 



A1 3 17-96 -1761 19-66 



F e ;0 3 407 -02541 .lowcvn 4 *46 



Feb 669 -0929) = 1237 Fe0 732 



MnO 0-29 -0041 032 



MgO 5-33 -1332 584 



CaO 7-66 -1368 S-39 



K 1-37 0146 1-50 



Na 3-62 -0584 3-96 



S " 0-12 -0037 013 



F 0-04 -0021 0-04 



C0 2 6-34 -1441 



H 2 above 100° C. . 1-38 -0767 



H;Oatl00°C. ... 050 



Less O replaced by F — 0"02 ... - 002 



Totals 99-56 10000 



The carbon-dioxide was determined, by taking the difference 

 between the total loss on ignition and the amount of water present. 



Subsequently, an analysis was made of that portion of the rock 

 which is dissolved on boiling for a short time with 50 per cent, 

 hydrochloric acid. The alkalies were not determined, and the iron 



