Vol. 4] 



Knopf. — An Alteration of Serpentine. 



429 



Analysis of the silica-carbonate rock (excluding macroscopic 

 quartz veinlets) yielded the following result after digestion in 

 hydrochloric acid : 



Insol. residue = 16.24% (96.4%Si0 2 ) 



AlA f - ,J ~ b 



CaO= 13.20 



MgO= 25.89 



Theoret. % C0 2 - = 38.61 



Total H,0 = 1.15 



Sum = 100% 



The analysis represents the end product of two successive 

 processes : first, serpentinization, and secondly, dehydration and 

 carbonatization. The former involved a large increase of volume, 

 the latter a decrease. This latter process has been attended by 

 the formation of vitreous quartz veinlets, whose frequent comb- 

 in-comb structure indicate their deposition in open fractures. 

 The above analysis when compared with the composition of the 

 bronzitite shows several noteworthy features. An exact discus- 

 sion is not possible on account of the uncertainty of the volu- 

 metric relations involved, but the following points appear well 

 established: that there has been a large expulsion of silica by 

 carbonic acid ; that a notable enrichment of lime has taken place ; 

 and that the magnesia and total sesquioxides (largely iron) have 

 practically remained constant. 



It has already been pointed out that in thin section the silica- 

 carbonate rock often presents a highly unique and characteristic 

 structure, due to an anastomosing system of carbonate capillaries 

 enclosing discrete eye-like areas. These small lenticular areas are 

 composed of allotriomorphic carbonate, or spherulitic chalcedony, 

 or both together. 



The carbonate veinules possess a fibrous structure, the fibres 

 not being continuous across, but growing from both walls toward 

 the center. The center line is occupied by yellow iron oxide, 

 which frequently extends in between the carbonate fibres, thus 

 producing a sort of stiff, orthogenal dendritic structure. The 

 above phenomena represent a pseudomorphic replacement of a 

 schistose serpentine, the carbonate capillaries occupying the 



