14:0 G. H. Williams — Serpentine {Peridotite) occurring 



porphyritic structure. The irregular blood-red patches spoken 

 of by Vanuxera are scattered through this rock. The porphy- 

 ritic crystals here mentioned produce what Dr. Hunt has called 

 the "concretionary structure" of the serpentine. 



Under the microscope the rock of the first type appears as a 

 medium-grained aggregate of brown mica, octahedral crystals of 

 both a yellow transparent, and of an opaque black mineral, green 

 or colorless serpentine and carbonates. Occasional larger crys- 

 tal-forms are scattered through this matrix. It is remarkable 

 that, in spite of nearly all the stibstance of this rock being 

 secondary, the original structure is almost perfectly preserved. 

 Sharply defined crystal-forms show by their shape that they 

 must have once been olivine or enstatite, although none of the 

 original substance now remains except rarely in the case of 

 enstatite. The mica is that peculiar brown biotite which is 

 well known to be characteristic of the eruptive peridotites.* 

 It is without crystal-form, and often shows a peripheral zone of 

 a lighter color due to bleaching. The minute octahedral crys- 

 tals mentioned above are from '05 to -005mm. in diameter. 

 The opaque black ones are for the most part chromite, although 

 some of them may be magnetite, as the magnet shows that there 

 is a little of this substance in the rock. If the serpentine 

 powder be treated with hydrofluoric or with concentrated sul- 

 phuric acid, everything is dissolved except these small black 

 octahedrons, which give a strong chromium reaction in both 

 the phosphorous and borax beads. There is therefore little 

 doubt that they are mostly chromite. 



The little transparent yellow crystals entirely disappeared 

 when the rock was treated with either of the above mentioned 

 acids, and the solution in each case gave a strong titanium 

 reactiou with H„0 2 . This, together with the fact that they 

 showed an isotropic character when examined in polarized 

 light, suggested the possibility of their being perofskite, and such 

 upon further investigation proved to be the case. In spite of 

 the frequently repeated assertion in mineralogies that perofskite 

 is not attacked by acids, it is well-known that it is dissolved by 

 concentrated sulphuric acid, as stated by Danaf and Rosen- 

 busch4 That it must also be attacked by hydrofluoric acid is 

 shown by the fact that both Baumhauer§ and Ben Saude| 

 used this reagent in a dilute form to produce etched figures on 

 the planes of perofskite crystals. Stelzner^f also found that it was 

 impossible to isolate the minute octahedrons of perofskite from 



* Vid. Rosenbusch: Die massigen Gesteine, 2d ed., 1886, p. 259. 



-(• System of Mineralogy, 5th ed., p. 146. 



% Die petrographisch wichtigen Mineralien, 2d ed., p. 295. 



§ Zeitschrift fur Krystallographie, Bd. iv, p. 187, 1880. 



[Ueber den Perowskit, Gottingen, 1882, p. 22. 



^[ Neues Jahrbuch fur Mineralogie, etc., II Beilage Bd., p. 392. 



