156 THE VOYAGE OP H.M.S. CHALLENGER. 



formed into white material, it may even amount to 90 per cent. The substance which 

 fills the crystals of plagioclase in this rock is thus silica. The augite sections even 

 have not escaped this alteration : their margins appear corroded ; a zone of silica, like 

 that which we have observed in the felspars, surrounds them as with a frame, and sends 

 ramifications through the crystals until, in many cases, the augite is transformed into a 

 greyish isotropic mass. The augite can only be recognised by its external form, which 

 is generally preserved, or by greenish or brownish fragments entirely embedded in 

 silica. The vitreous ground-mass itself is subject to a similar modification in some 

 cases, its usual yellow colour passing into grey. The outlines of the microliths are made 

 indistinguishable, except perhaps in the case of magnetite, and all the constituent minerals 

 seem to be embedded in the opaline mass. The siliceous matter rarely assumes the form 

 of quartz, but here, as at Ternate, granules are sometimes seen possessing the optical 

 properties of that mineral, or of tridymite. Quartz or tridymite is detected most fre- 

 quently in the fragments covered with a coating of more or less powdery white material. 



The alteration and displacement of these minerals by siliceous matter must be 

 caused by the action of gaseous volcanic emanations, by jets of steam, and by high 

 temperature. Amongst the vapours which attack silicates most energetically are those 

 of hydrochloric and sulphuric acid. The latter, detected in the fumaroles of Goonong 

 Api, can easily remove all the bases of this lava as soluble sulphates, which would readily 

 be washed away. This is the case with the alumina and iron, while the silica, with 

 which they were combined in the eruptive rocks, remains alone in the form of hydrate. 



The alteration of felspar and augite into a substance resembling opal is a fact 

 observed elsewhere. We may refer, for instance, to the investigations of Eammelsberg 1 

 on the pyroxene of Vesuvius in the lava of 1852, in which the amount of silica reached 

 85"34 per cent. ; water was present to the extent of 5'47 per cent. ; the mineral which 

 had been altered by the action of fumaroles contained only traces of bases. Morawski 

 and Schinnerer 2 showed that the sanidine of the trachyte from a solfatara near 

 Pouzzolie contained 90'19 per cent, of silica and 4"19 per cent, of water. According 

 to Blum, 3 the sanidine of Furnas is similarly changed into opal, the surface of the 

 crystals remaining hard, while the interior is cellular and porous. Finally, Fritsch and 

 Eeiss * found the same modification in the felspar of a phonolitic rock of Pico de Teyde. 

 These facts bear the most perfect analogy to those we have been describing, and they 

 should be attributed to the same cause. The presence of quartz and tridymite, 

 which were detected in some of the altered rocks, may be explained by their formation 

 as products of sublimation, a mode of origin for these minerals too well known to 

 require to be discussed here. 



1 Rammelsberg, Pogg. Ann., Bd. xlix. p. 388. 



- Morawski and Schinnerer, Verh. geol. Reichsansta.lt, p. 161, 1872. 



3 Blum, Die Pseudomorphosen des Mineralreichs, Bd. iii. p. 52. 



* Von Fritsch und Reiss, Geologische Beschreibung der Insel Teneriffe, p. 423, 1868. 



