342 ORIGIN AND DEiNELOPMENT OF ORE-DEPOSITS. 



Serpentinization. This is a change somewhat analagous to 

 the last, to which olivine, and in a less degree most forms of 

 pyroxene and amphibole, as well as some other minerals, are 

 subject. Full and free circulation of the solvent seems to be 

 necessary, and probably also both heat and pressure. I endea- 

 voured to show some years since that " serpentinous change is 

 scarcely less common, though of course much less extensive in 

 the West of England, than kaolinization, and that in some 

 instances the same rock-mass affords evidence of both kinds of 

 change," and after quoting from Dr. Sterry Hunt, I went on to 

 say " The change has probably required a circulation of waters 

 containing magnesian chloride in solution, and aided by heat and 

 pressure ; submergence in the waters of the sea to a considerable 

 depth would suffice to give all these conditions."* 



Uralization and amphiholization. The changes of augite into 

 uralite, and of uralite and diallage into hornblende are so-called. 

 They are almost as universal in the West of England as kaolini- 

 zation, although their field of operation is far less extensive. 

 The changes, too, are very analagous, except that there is no 

 evidence of the presence of fluorine, while considerable heat and 

 pressure were probably essential in addition to the circulating 

 solutions. Most of the intrusive " greenstones " and " gabbros" 

 afford evidence of this kind of change. f 



Schillerization or the alteration of augite, enstatite, &c., into 

 diallage and bronzite, consists (according to Prof. Judd, who has 

 made a special study of the subject), in the development of 

 minute enclosures in the form of thin plates or delicate rods 

 along one or more sets of parallel planes in the minerals so 

 altered. This development, which is very common in the Lizard 

 district, always seems to take place along definite cleavage planes, 

 but the largest development is not always in the freest cleavages. 

 These plates, rods, &c., are considered by Prof. Judd to be 

 " negative crystals filled with products of decomposition." The 

 development seems to be proportionate to the depth of the rock 

 from surface at the time of the change, and consequently may 



^Geological Magazine, July, 1885, August, 1886, and May, 1887. " The Geo- 

 logical History of the Oornish Serpentinous Eocks." 



t See Allport, " Rocks of Land's End District," Quar. Jour. Geol. Soc. 1876, 

 pp. 15-29. Phillips, Quar. Jour. Geol. Soc, May, 1876. 



