490 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



hand, in the portions composed of labradorite and andesine 

 are glass-clear, like sanidine, except in patches or " strings" 

 (widened cracks) filled with decomposition products. Of 

 these the most abundant is analcite, generally clear ; there 

 is also a colourless mineral, in small flakes and granules, 

 showing usually one good cleavage and straight extinction 

 whose mean R.I. is about 1*55 or 1*56, and whose D.R. is 

 about 0*025, which is doubtless a scapolite, for the R.I. is 

 distinctly too low for it to be prehnite ; there is serpentine 

 in radial patches and tufts derived from neighbouring olivine 

 and chlorite forming a mosaic of minute radial bundles. 



The relative proportions which the rock contains respec- 

 tively of calcic plagioclase and soda orthoclase, including 

 in the latter the alkaline mantles of the plagioclase crystals, 

 is not easy to determine. It was impossible, owing to the 

 advanced state of decomposition, to separate them in the 

 Rosiwal measurement, but as a rough estimate we would 

 distinguish as alkali felspar from a quarter to one-third of the 

 total felspar, i.e. from 12 to 17 per cent, of the whole rock. 



In addition to the decomposition products already men- 

 tioned above, there is only one other of importance, a zeolite 

 in radial spherulitic aggregates approaching \ mm. in 

 diameter. Its fibres have a low extinction angle, its R.I. 

 is 1*50 and its D.R. about '008, which properties suggest 

 stilbite. The serpentine in cracks and cavities is a light 

 olive green in colour; where it replaces olivine, which has 

 entirely disappeared, its colour is deep reddish-brown. 



Apatite is abundant and occurs in large needles, some- 

 times 2 mm. in length, penetrating augite and felspar 

 crystals, but only the edges of the olivine pseudomorphs, and 

 very small in the centres of the ilmenite crystals. All the 

 iron ore has the form and colour of ilmenite, but may be 

 magnetiferous and is unaltered. 



Order of Consolidation. The most difficult, but most 

 interesting, point in the sequence of consolidation is the 

 relation of the periods of crystallisation of the felspars to 

 one another and to augite. Each mineral of this group 



