from GougKs Island, South Atlantic. 383 



cryst, and as the final products just before consolidation globu- 

 lites and small sanidine microlites. 



The olivine occurs very rarely indeed. It is fresh and of a 

 pale yellow color and shows magnetic resorption. That it is 

 indeed olivine is shown by its optical behavior, its high double 

 and single refraction, its extinction parallel to two cleavages at 

 about right angles, one of which is more pronounced than the 

 other, and its general outline and habit. 



This was confirmed by treatment of the powdered rock by 

 hydrofluoric acid according to Fouque's method of isolation 

 and in the residue the lens disclosed a number of the olivines 

 which were carefully picked out and subjected to qualitative 

 analysis. This proved the mineral to be a silicate soluble in 

 hydrochloric acid and in the solution were only iron and mag- 

 nesia, lime not being present. The mineral is therefore un- 

 doubtedly an olivine and one rich in iron. The method of 

 isolation also showed that it was very scarce in the rock. The 

 crystals attain a size of \5 to l mm . 



The presence of olivine in trachytic rocks is not unknown. 

 It is mentioned by Miigge* as occurring in the trachytic lavas 

 of the Azores, by Michel Levyf in augite trachyte of Monte 

 Dore which is however of an andesitic type, and by Fuchs;}: 

 as an accessory component of the trachyte of the Arso stream 

 on the island of Ischia. Its occurrence in the present case in 

 a magma containing less than "5 per cent of magnesia is, how- 

 ever, remarkable and to be paralleled with its presence in the 

 rhyolite of Iceland as shown by Baekstrom,§ and that of fay- 

 alite in the rhyolitic obsidian of the Yellowstone Park,|| and of 

 the Lipari Islands^" as shown by Iddings. The latter author 

 parallels such occurrences with that of sporadic quartz grains 

 in basalt** and refers their origin to the mineralizing tendency 

 of included water vapor in highly heated magmas under con- 

 ditions of great pressure. In the present case the corroded 

 state of the olivines evinces that in the later chemical and phys- 

 ical conditions under which the magma consolidated, the com- 

 pound as a chemical one was incapable of existing and that it 

 was therefore formed under different ones. The fact that the 

 magma chilled as a glass proves that it was ejected and cooled 

 with comparative rapidity, and it is reasonable to infer that 

 such ejection through comparatively long and narrow conduits 

 must be accompanied by varying degrees of pressure and with 



* N. J., 1883, ii, pp. 217. 



\ Bull. Geol. Soc. France, 3d series, xviii, pp. 812. 

 JTschermak Mitth., 1872, pp. 224. 

 § G-eol. Foren. Forhandh., xiii, No. 7, pp. 644. 



|| This Journal, vol. xxx, p. 58, 1885; cf. also 7th Ann. Rep. TJ. S. G. Surv., 

 pp. 270, 1888. TfThis Journal, vol. xl, p. 75, 1890. 



**This Journal, vol. xxxvi, p. 208, 1888. 



