348 ORIGIN AND DEVELOPMENT OF OKE-DEPOSITS. 



It is often supposed that silica has been deposited from acid 

 solutions. It is probable, however, that this is not the case in 

 general. The waters 6.o^^ ing from kaolin are always a little alka- 

 lien, while quartz-crystals are often found deposited on crystals of 

 calcite or chalybite, whose form and even whose lustre has not 

 been affected in any way.* 



Minerali%ation. This is a term used by miners in a some- 

 what arbitrary sense, to denote an impregnation of a rock with 

 iron-pyrites, copper-pyrites, blende, galena, or other sulphide 

 ore — or by the oxidized or decomposed indications of such. It 

 often happens that such mineralization, especially with the first- 

 named substance, exists over large areas, in regions where no 

 valuable deposits have been discovered. But the converse is 

 scarcely the case, since there are very few mineral deposits of 

 value known other than detrital deposits, which are not associ- 

 ated with rocks more or less mineralized. As in the cases of 

 calcification and silicificatiou there may be, and often is, evidence 

 of the direct introduction of the mineralizating substance from 

 without, while at other times there seems to have been a mere 

 re-arrangement of an original or pre-existing constituent. 



Pseudomorphism. In a large sense all such changes as 

 kaolinization, serpentinization, and the like are examples of 

 pseudomorphism. Technically, however, the term is limited to 

 chemical changes in minerals of definite form, either distinctly 

 crystalline, or at least exhibiting definite and recognizable 

 " imitative " forms. For instance, when crystals of pyrites are 

 found converted into limonite, calcite into cerussite, pyromorph- 

 i^e into galena, or felspar into kaolin, these would be at once 

 recognized as pseud omorphic. It is evident, however, that the 

 process is similar when extensive amorphous masses or dissemi- 

 nated grains are similarly changed. 



The gozzans and the china-clay and china-stone " deposits " 

 already mentioned are such examples, as much so as the altered 

 veins of iron carbonate at Pawton, the altered beds of iron 



* For some interesting experiments and instructive remarks on the decom- 

 position and re-construction of rocks by siliceous infiltrations, see a paper by 

 Alptonse Gages, read at the Geol. Soc. of Dublin, Nov. 18, 1858. {Bub. Nat. 

 Hist. Bev., Vol. vi.; 



