Notices of Memoirs — Notes on Rockall Island and Bank, 165 



Eockall is really of a very unique and interesting character. The 

 rock consists of three minerals — a soda-augite (segirine and acmite), 

 quartz, and albite. Accessory minerals, other than magnetite and 

 apatite, appear to be very rare in it. The proportion of these 

 ingredients is somewhat different in the several examples of the 

 rock, the darker-coloured variety containing more of the soda-augites. 

 The order of crystallization of the minerals of the rock is easily 

 determined. The soda-augite (gegirine and acmite) was the first to 

 separate, the centres of the crystals being grass-green segirine, and 

 the peripheral portion brown acmite, each mineral exhibiting its 

 characteristic pleochroism. After the augite, albite separated, and 

 the quartz crystallized out last. 



Apatite needles are found enclosed in all the minerals of the rock, 

 and among the secondary minerals of the rock are magnetite, 

 quartz, arfvedsonite, and a blue soda-amphibole. 



The specific gravities of different specimens determined by 

 Mr. H. S. Hinton, F.G.S., were found to vary from 2-71 to 2-94. 



An analysis of the rock of the highest specific gravity and 

 darkest colour, which was made by Mr. C J. S. Makins, under the 

 superintendence of Professor W. A. Tilden, gave the following 

 result : — 



Silica 73-60 



Alumiua ... ... ... ... ... 4'70 



Ferric Oxide 13-10 



Manganese Oxide ... ... ... ... -93 



Nickel Oxide -06 



Magnesia ... ... ... ... ... '11 



Lime -37 



Soda 6-96 



Potash ... ... ... ... ... ... minute trace 



Phosphoric Acid ... ... ... ... traces 



99-83 

 This analysis shows the miineralogical constitution of the rock 

 to be as follows : — 



^girine and Acmite ... ... ... 39 per cent. 



Quartz 38 ,, 



Albite 23 „ 



(With a little anorthite.) 



The soda-augite (segirine and acmite) appear to have a very 

 similar composition to that of the rocks of the Ghristiania district 

 described by Brogger, while the bulk analysis of the rock resembles 

 that of some of the quartz ceratophyres and some of the quartz- 

 pan tellerites of Foerstner. 



The nearest analogues of the rock of Rockall are to be found 

 among the grorudites of Brogger (quartz-tinguaite of Rosenbusch). 

 Professor Brogger, who has examined a section of the rock sent to 

 him, agrees that the rock is similar to his grorudites in mineral 

 composition, but he demurs to it being included under his name on 

 account of the differences in structure. On the other hand, the 

 rock of Rockall is shown to differ from the Natron-granites, some 

 of which it resembles in structure, by its high proportion of augite. 



