Prof. T. G. Bonney — On tJie Rauenthal Serpentine. 69 



whole I have no doubt that in my slides the peridotic constituent is 

 distinctly in excess. Hence either our specimens are exceptional, 

 or Herr Weigand has examined slides cut from an exceptional 

 specimen, or he has failed to distinguish the two minerals. So far 

 as I can form an opinion from the woodcut given in his paper, I 

 confess that I incline to the last solution, for I think that even there 

 I can see indications of a difference in the constituents. In any case 

 it is, I think, clear, that the Kauenthal serpentine is only a variety of 

 the group of serpentines derived from the alteration of olivine-horn- 

 blende rocks, of which we have excellent examples at Mullion, 

 Pradanack, and other localities at the Lizard.^ To some of these 

 indeed the Eauenthal rock has, both macroscopically and microscopi- 

 cally, a considerable resemblance. All that we can assert is that 

 occasionally (possibly) ,the white hornblende is rather exceptionally 

 abundant, and that (as sometimes occurs elsewhere) a peculiar variety 

 of chlorite is locally developed. 



I may take this opportunity of calling attention to another state- 

 ment on p. 119 of Mr. Teall's work, which I believe to be not quite 

 accurate. Speaking of the occurrence of felspar in the Lizard ser- 

 pentines, Mr. Teall says, " It is present also in slides of a variety of 

 serpentine from the liill Head, near Kynance, lent to the author by 

 Mr. Waller," to which he appends the note, "The author has recently 

 visited the district and ascertained that serpentine containing felspar 

 forms a large portion of the Eill Head. The original rock, therefore, 

 was of the nature of a picrite." It would not, of course, be sur- 

 prising if a little felspar occurred in a mass of serpentine, and I had 

 already noticed at Gue Graze (on the same coast further to the north) 

 a decomposed mineral which I thought might be felspar, as is men- 

 tioned by Mr. Teall in the passage from which I have quoted, though 

 it would be more correct to sa}^, " Professor Bonney records it 

 (felspar) as [probably] occurring at Gue Graze," as will be seen on 

 reference to my description of the Gue Graze rock (Quart. Journ. 

 Geol. Soc. 1877, p. 918). But in regard to the Eill rock I do not con- 

 sider the mineral mentioned by Mr. Teall to be felspar. No doubt 

 it occasionally closely resembles a plagioclastic felspar, both in its 

 earthy decomposition and its lamellar twinning ; but after carefully 

 examining my own slides (including one cut from a specimen kindly 

 given me by Mr. Teall, and one which Mr. Waller freely placed at 

 my disposal, I am distinctly of opinion that the mineral is not felspar. 

 This earthy decomposition (which weighed much Avith me in ex- 

 amining the Gue Graze rock) is, I now find, not unfrequent iu 

 colourless minerals belonging to the pyroxene group, and the mode 

 in which the transparent parts of the crystal are attacked appears to 

 me to be different from that in which kaolinization proceeds in a 

 felspar. There are also some minute structural peculiarities in the 

 mineral, which to my eye recall a member of the above-named group 

 and not a felspar. The twinning, which however is not very common, 

 is no doubt curiously like that of a plagioclastic felspar; but similar 

 twinning does occur, though rarely in augite or diallage (Fouque and 

 1 As mentioned in my paper, QJ.G.S. vol. xxsix. p. 23. 



