OCCURRENCE AND ALTERATION OF GLAUCONITE. 351 



Talc forms in the upper zone of metamorphism. In this respect it is 

 like chlorite and serpentine. It is especially likely to form under condi- 

 tions of weathering. The minerals from which talc is derived are as 

 follows: Actinolite, andalusite, anthophyllite, bronzite, cyanite, diopside, 

 enstatite, gehlenite, hypersthene, muscovite, olivine, phlogopite, pyrope, 

 sahlite, scapolites, sillimanite, spinel, staurolite, topaz, and tremolite. The 

 manner of formation is given under the various minerals. Of these minerals 

 the more important are the nonaluminous amphiboles and pyroxenes, both 

 orthorhombic and monoclinic. It also forms rather abundantly from olivine, 

 mica, and garnet, 



Alterations. — Alterations of talc are not recorded. It appears to be one 

 of the end products of rock alteration in the belt of weathering. However, 

 I have no doubt that when the talc-bearing rocks are buried so deeply as to 

 pass into the zone of anamorphism and there alteration takes place, talc, 

 like chlorite, serpentine, and other minerals, is destroyed, and that from it 

 alone, or from it and other minerals, the classes of heavy minerals from 

 which the talc was originally produced are ag'ain formed. 



GLAUCONITE. 

 Olauconite: 



Essentially a hydrous silicate of iron and potassium. Definite formula unknown. The 



potassium ranges from 1.85 to 6.56 per cent. 

 Amorphous. 

 Sp. gr. 2.2-2.4. 



■■■ — Glauconite occurs in sediments of many kinds and ages. 

 Where it is so abundant as to give the rock a green color it is known as 

 greensand. Greensands are especially prevalent in the Cretaceous. 



Alterations. — -Since no definite formula for glauconite can be given it is 

 impracticable to write equations representing the transformations. But 

 since glauconite is almost, if not quite, unknown in the schists and gneisses 

 formed in the zone of anamorphism it seems certain that under the condi- 

 tions of that zone the glauconite is broken up, its constituents passing into 

 other minerals. 



