542 w. w. WATTS o:^ the igxeoits and 



In some of the specimens minute short needles of green mineral 

 occur, arranged parallel to the pinacoid ; in others they are longer 

 and look more like distinct twins ; exactly similar needles are fre- 

 quently sown broadcast through the ground of the rock. They are 

 somewhat opaque, and it is difficult to determine anything definite 

 about them ; they appear to be dichroic and to have a high extinc- 

 tion-angle, so they may possibly be an abnormal form of augite. 



The hypersthene described by Mr. Whitman Cross * shows pris- 

 matic cleavage ; that noticed by Mr. Teall from the Cheviots f 

 appears to have no pinacoidal cleavage ; the hypersthene in a Mont- 

 serrat lava described by Mr. Waller J, too, shows prismatic as well 

 as a brachypinacoidal cleavage ; and all these are different in colour 

 and in polarization-tints from the mineral here described. On the 

 other hand the enstatite of Eycott Hill, described by Prof. Bonney §, 

 is like this in shape, colour, cleavage, fibrous structure, and altera- 

 tion, so that the mineral is some form of enstatite or bronzite. 

 Prof. Hosenbusch too speaks of a similar mineral with pinacoidal 

 cleavage. 



(b) Monodinic pyroxene. In the best-preserved specimens there 

 is often sufficiently preserved to be recognizable a considerable 

 quantity of augite, though the enstatite predominates over it. It is 

 often altered to greenish chloritic products, and even further to 

 calcite. 



3. Amphihole. In one or two of the slides there is pretty cha- 

 racteristic brown dichroic hornblende, and in many of them are 

 elongated and basal sections which, though very highly altered, must, 

 from their outline, be referred to this mineral. 



4. Magnetite, Ilmenite, and Hamatite, are all of them present, the 

 second probably in greatest quantity, in distinct crystals generally 

 of a fair size though often in quite minute grains. Haematite is 

 common in the felspars and pyroxenes where the rock is in an 

 exposed situation, and tints the crystals and cracks with its charac- 

 teristic colour. 



5. Black mica has been suspected by Prof. Bonney in at least one 

 of tbo slides, but it is evidently a rare accessory. 



6. Apatite occurs, too, in several of the specimens, in clear elon- 

 gated needles, and is evidently a product of early consolidation. 



7. There are many alteration-products, mostly of a serpentinous 

 nature, but some are clear and bright and colourless. 



The ground-mass of the rock is a close felt of colourless micro- 

 liths apparently of felspar ; it breaks up into a pale mosaic between 

 crossed nicols ; amongst these are innumerable opaque, whitish and 

 greenish bodies, whose presence renders it exceedingly difficult to say 

 whether the base has any glass left in it. I have been unable, after 

 the most minute search, to detect any unindividualized glass in the 

 matrix, though in a few base inclusions in the felspar I think there 



* W. Cross, he. cit. p. 21. 



t Teall, Geol. Mag. dec. ii. vol. x. p. 103. 



+ Waller, ihid. p. 291. 



§ Bonney, Geol. Mag. dec. iii. vol. i. p. 77. 



