98 THE NATUEALIST IX NICARAGUA. [Ch. VI. 



superheated steam ; and M. Angelot has suggested that 

 fused rock under great pressure may dissolve large 

 quantities of the vapour of water, just as liquids dissolve 

 gases. The presence of the vapour of water would 

 cause the liquefaction of quartz at a much lower tempe- 

 rature than would he possible hy heat alone, unaided hy 

 water.* I know that this opinion is contrary to that 

 usually held hy geologists, the theory generally 

 accepted being that mineral veins have been produced 

 by deposits from hot springs ; but during twenty years 

 I have been engaged in auriferous quartz mining in 

 various parts of the world, and nowhere have I met 

 with lodes, the phenomena of which would be explained 

 on this hypothesis. The veinstone is pure quartz con- 

 taining water in microscopical cavities, as in the quartz 

 crystals of granite, but not combined as in the hydrous 

 siliceous scoriae deposited from hot springs. The lodes 

 are not ribboned, but consist of quartz, jointed across 

 from side to side, exactly like trappean dykes. There is 

 often a banded arrangement produced by the repeated 

 re-opening and filling of the same fissure ; but never, in 

 quartz veins, a regular filling up from the sides towards 

 the centre, as in veins produced by deposits from 

 springs. Quartz veins extend sometimes for miles, and 

 it is necessary to suppose on the hydro-thermal theory 

 that they kept open sufficiently long for the gradual 

 deposition of the veinstones, without the soft and shat- 

 tered rocks at their sides falling in, nor yet fragments 

 from above ; although there are many lodes, fully twenty 

 feet in width, filled entirely with quartz and mineral 

 ores, without any included fragments of fallen rocks, 

 * K. C. Sorby, Jour. Geol. Soc. , vol. xiv. 



