258 



PEOF. T. G. BONJSTEY ON HORNBLENDE-PICRITE 



from Giimbel, ' Geognostiche Beschreibung des Fichtelgebirges,' 

 p. 152 ; then ' palseopicrite ' analyzed by Alois Gamroth, 'Jahrbuch 

 k.-k. Reichsanst.' 1877, Min. Mitth. p. 278; the last, the Schriesheini 

 rock, analyzed by Puchs, ' N. Jahrbuch,' 1864, p. 326. 



By a comparison of these analyses with those made by Mr. J. A. ' 

 Phillips, we see that the latter have slightly more silica, considerably 

 more alumina, and less magnesia than the two rocks described by 

 Prof. Giimbel, the composition of which more nearly corresponds 

 (except that there is less water) with that of a normal serpentine, 

 from which they differ chiefly in a lower percentage of magnesia and 

 a higher one of lime. Even these, however, differ in the ratio of the 

 silica to the magnesia, which in the Schwarzenstein rock is about 

 13 : 10, in the Holler 16 : 10, while in such a rock as the serpentine 

 of the Lizard, Cornwall, it is roughly 11 : 10. In the first rock from 

 Sohle it is about 15 : 10. In the Welsh rock it is rather more than 

 26 : 10 ; but then this ratio is exceeded by two of Prof. Tschermak's 

 specimens, while in the Schriesheim rock it is 22*5 : 10. 



This last has also been called a " Schillerfels " and an olivine- 

 diallage rock ; but the conspicuous mineral is not bastite, and my 

 specimen contains little, if any, normal diallage. 



Por comparison I append one or two analyses of serpentines and of 

 olivine rock. 





I. 



II. 



III. 



IV. 



Si0 



38-86 

 2-95 

 1-86 

 5-04 



trace 

 0-28 

 0-08 



trace 



34-61 

 0-33 

 0-77 



1552 



38-58 

 3-06 

 1-95 

 510 



trace 

 0-30 

 0-08 



trace 



34-32 

 0-30 

 076 



15-52 



38-50 

 102 

 4-66 

 3-31 



0-59 



1-97 



36-40 



j 1-37 



12-35 

 0-41 



43-84 

 114 



876 

 0-12 

 0-51 

 0*42 

 1-71 

 44-33 



106 



ALO, 



Fe 2 3 



FeO 



MnO 



MO 



Cr o 0, 



CaO 



MgO 



K 2 



Na 2 



H 2 



FeS 







10030 



99-97 



100-58 



101-89 



I. & II. Duplicate analyses of a Lizard serpentine by Mr. J. A. Phillips (Phil. 

 Mag. Feb. 1871). 



III. Analysis of a serpentine from near Cadgwith by Mr. Huclleston (Q. J. G. S. 



vol. xxxiii. p. 925). 



IV. Olivine rock from St. Paul's Island (Senard, ' Challenger Voyage,' Narrative, 



vol. ii. p. 7). 



In short, an examination of the former analyses shows that the 

 chemical composition of the rock picrite is rather variable, and that, 

 while it lies closer to the normal peridotites than to any other, it is, to 



