354 A TREATISE ON METAMORPHISM. 



bination of chlorite and talc, the aluminous part of the original molecule 

 going to the chlorite and the nonaluminous part into the talc. The 

 development of epidote is largely if not wholly confined to the belt of 

 cementation. But chlorite apparently forms both in the belt of cementa- 

 tion and in the belt of weathering, especially where there is abundant 

 vegetation. Under extreme and long-continued conditions of the belt of 

 weathering the serpentines, chlorites, and epidotes are likely to further 

 degenerate, breaking up into carbonates of the alkalies and alkaline earths, 

 • anhydrous or hydrous oxides of aluminum and iron, quartz, and kaolin. 

 Or in the belt of weathering these end products may directly develop from 

 the metasilicates without the serpentines, tjhlorites, and epidotes as inter- 

 mediate products. In the zone of anamorphism the pyroxenes change to 

 amphiboles; the pyroxenes and amphiboles both alter to biotite; the 

 olivines change to the amphiboles, anthophyllite, tremolite, and actinolite. 

 The biotite group does not ordinarily alter. But by profound metamor- 

 phism the material of the biotites, amphiboles, pyroxenes, etc, may pass into 

 the still heavier class of minerals represented by the garnets, staurolites, etc. 



THE TITANATES. 

 TITANITE AND PEROVSKITE. 



As rock-making constituents only two titanates of importance, titanite 

 and perovskite, are here treated, ilmenite being given under the oxides. 



Titanite: 



CaTiSi0 5 . 

 Monoclinic. 

 Sp. gr. 3.4-3.56. 



Perovskite: 

 CaTi0 3 . 



Isometric or pseudoisometric. 

 Sp. gr. 4.017-4.039. 



TITANITE. 



occurrence. — Titanite occurs as a rather subordinate but widespread min- 

 eral as an original pyrogenic constituent of igneous rocks, and also in the 

 schists and gneisses. So far as observed, titanite as a secondary constituent 

 is derived from ilmenite and rutile. These alterations are discussed under 

 those minerals. 



