ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 341 



Kaolini%ation is a peculiar change to which, many aluminous- 

 alkaline silicates are liable, and especially the various forms of 

 felspar. It seems to be effected by the underground circulation, 

 which, decomposing the original silicates, carries off the alkalies, 

 leaving a hydrated silicate of alumina behind, free quartz being 

 at the same time deposited whenever there are cavities. This 

 change is effected gradually, so that in many instances the form 

 of the original felspar is perfectly retained. Few felsjDars are 

 altogether without indications of, at least, incipient kaolinization ; 

 and the china-stone (Petuntzyte) and china-clay rock (Carclazyte) 

 so abundant in the West of England are important results of 

 this change operating on an extensive scale. 



It seems likely that, generally speaking, f elspathic rocks may 

 have been kaolinized by the action of percolating atmospheric 

 waters charged with carbonic acid, and at ordinary temperatures 

 and pressure ; but it is very unlikely that any kaolin of economic 

 importance has been thus produced. Rather, in such instances, 

 the change seems to have been effected by the action of solutions 

 containing fluorine (with sometimes at least chlorine and boron) 

 arising from considerable depths through fissures, as was long 

 ago suggested by Von Buch, Daubree, and others. All forms of 

 circulation could aid effectively in this ; the canalicular to bring 

 up the active solvent from below, as well as to carry off the 

 dissolved alkalies, and the capillary and interstitial to permeate 

 add change the interior of the rock-substance. 



The enormous economic importance of this form of change 

 in the felspathic rocks of the West of England is indicated by 

 the fact that the yearly exports of china-clay and china-stone 

 from the two counties now amount to over 400,000 tons, necessi- 

 tating the removal of probably 6 million tons of rock and 

 overburden. The softening of aluminous killas near many lodes 

 and the production of prian are direct results of this kaoliniza- 

 tion process ; while the production of schorl-rock, gilbertite 

 quartz, and capel, are intimately connected with it.* 



*For a fuller discussion of this subject, see the following works by the present 

 author : — 1, "The Hensbarrow Granite District," Truro, 1878. 2, "On Cornish 

 Tin Stones and Tin Capels," Truro, 1883. 3, "On the Nature and Origin of 

 Clays," Min. Mag., 1888. 



