OKIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 343 



be regarded as the result of what I have called capillary circu- 

 lation, aided by heat and pressure. It is probable that some 

 special solvent is necessary, since schillerization is local and not 

 general, and also because the secondary deposits in the negative 

 crystals seem to be in some instances derived from without. This 

 schillerization is common in many of the Lizard rocks, but I am 

 not aware it has been observed in the mining districts of the 

 West of England. The conversion of orthoclase into murchi- 

 sonite, as observed near Dawlish, seems to be a combination of 

 kaolinization and schillerization, and the Y^eculiav chafoi/ant lustre 

 of labradorite is, according to Judd, due to a similar development 

 of ?</i!rffl-microscopic plates.*' 



Alunation. This consists in a decomposition of alkaline 

 aluminous silicates by the aid of sulphuric acid. The acid usually 

 results from the atmospheric oxidation of iron pyrites, as may be 

 seen in the shallower parts of many of our mines, and in some 

 places on the coast, in which case it may be regarded as a form 

 of weathering,! or it may be supplied by volcanic fumeroles, or 

 by thermal springs. Alunation may be seen in operation on 

 many mine-burrows where pyrites occurs associated with 

 aluminous veinstones. ;]; 



Calcification. Where calcareous matter is contained in the 

 rocks, the waters flowing over or issuing from them are always 

 ''hard," that is, they contain carbonate of lime in solution. 

 When such waters flow over loose sands or porous strata as met 

 with along the north coast of Cornwall, the interstices become 

 filled with carbonate of lime in a more or less crystalline condition. 

 This may be seen in the calcareous amygdaloids near Port Isaac, 

 on the north coast ; in the slates underlying limestones in many 



* For a full discussion of schillerization, see Judd, Quart. Journ. Geol. 8oc. 

 163, pp. 377, 383, 387, 408 ; and 165, p. 82. 



t See the account of the hot chamber in the author's description of the 

 Perran Iron Lode already referred to. 



X An interesting example of alunation on a large scale is afforded by the 

 Yorkshire cliffs near Whitby, and by the cliffs of London Clay at Sheppey. 

 Daubree, in his Les eaux anciennes, also refers to an instructive example at 

 Tokay, in Hungary, where certain trachytic tuffs have been converted by the 

 agency of thermal springs containing sulphuric acid into alunite. This material 

 forms extensive fossiliferous beds at the foot of the eruptive masses, while the 

 silica set free has super-silicated the tuffs in question so that they become suitable 

 for millstones. 



