EPIDOTB 



23 



well-defined crystals and also in irregular grains, either singly or 

 grouped in peculiar clusters to which the name polysomatic has 

 been applied by Tschermak. The serpentinous alteration takes 

 place along the irregular curvilinear lines of fracture, and under 

 favorable conditions continues until the transformation is com- 

 plete. The following analyses by Helland, as quoted in TealFs 

 British Petrography, illustrate the simplicity of the chemical 

 changes which here take place : 



Analyses showing Change of Olivine to Serpentine. 



SiOa . 

 AlfiOg , 

 FeaOs. 

 CrO , 

 MgO - 

 H2O 



41.32% 



0.28 



2.39 



0.05 

 54 69 



0.20 



98.93% 



n 



42.72% 



0.06 



2.25 

 Trace 

 42.52 

 13.39 



100.94% 



III 



4348% 



43.48 

 13.04 



100.00% 



T. Olivine, Snarum, Norway. 11. Serpentine derived from the same. 

 III. The theoretical composition of serpentine. 



Aside from the assumption of some 13% of water, the princi- 

 pal change, as will be noted, is a loss in magnesia which as a 

 rule separates out as a carbonate. The iron, which existed as 

 protoxide, is further oxidized and crystallizes out along lines of 

 fracture as magnetite or hematite, or in the hydrous sesquioxide 

 form known as limonite. Through decomposition, a portion or 

 all of the silica may be set free as opal or chalcedony, the mag- 

 nesia going over to the condition of carbonate, and the iron 

 passing into various hydrated oxide forms such as are most 

 stable under the existing circumstances. 



E-pidote, — Composition: Silica, 37.83%; alumina, 22.63%; 

 iron oxides, 15,98% ; lime, 23.27% ; water, 2.05%. 



This mineral is a common constituent of many granites, 

 gneisses, and schists, especially the hornblendie varieties. It 

 is particularly abundant, however, as a secondary constituent 

 in basic eruptives, where it results from the alteration of the 

 original ferromagnesian constituents such as the augites, horn- 

 blendes, or micas. It is the presence of this mineral or a see- 



