2 PROCEEDINGS OF THE NATIONAL MUSEUM vol.69 



glauconite of southeastern Missouri occurs in the Bonneterre dolo- 

 mite of Upper Cambrian age, and is intimately associated with 

 the lead ore, and locally forms 50 per cent or more of the rock. 

 The grains vary in size and shape, and the relations to the inclos- 

 ing rock have possibly been modified by recrystallization of the 

 dolomite. The type of glauconite which is most abundant forms 

 loose aggregates that conmionly occur as more or less rounded 

 grains. Another type forms wedge-shaped areas that are sharply 

 bounded by euhedral dolomite crystals. Some of the glauconite 

 in diamond drill cores from the St. Joseph Lead Co. mines forms 

 small compact grains about 0.2 millimeter in diameter in dolomite 

 and associated with a little detrital quartz. The individual grains 

 of all these types are never truly amorphous and they leave no 

 doubt of their crystalline character. They are composed predomi- 

 nantly of the fine grained aggregates of overlapping crystal plates 

 that are characteristic of nearly all glauconites, but a large pro- 

 portion of the grains in the compact type of glauconite from the 

 St. Joseph Lead Co.'s drill cores are single crystal individuals, and 

 an occasional grain of the other types shows this property. 



A very careful study of the greensands of New Jersey has been 

 made by Doctor Mansfield,* of the United States Geological Survey, 

 who describes and figures glauconite grains, each of which is a single 

 crystalline individual. Some of the New Jersey material was placed 

 at the disposal of the writer by Doctor Mansfield and the optical 

 properties of this glauconite have been determined. 



Glauconite occurs in the bentonitic horizon of the Ordovician of 

 Tennessee, Alabama, and Kentucky that has been described by 

 Nelson.^ At Singleton, Tenn., it is associated with quartz, feldspar, 

 muscovite, and the claylike mineral, montmorillonite, that is charac- 

 teristic of bentonite. A few of the glauconite grains have the struc- 

 ture of the usual cryptocrystalline aggregate, but a majority of them 

 are single crystal individuals. Some crystals of glauconite lie be- 

 tween the lamellae of muscovite plates, and others are imbedded in 

 montmorillonite crystals, and crj^stals of glauconite are often in 

 parallel orientation with muscovite and montmorillonite. 



Glauconite has also been studied from several other localities with 

 determination of the optical properties. Individual crystals of 

 glauconite were found aft^r careful search in almost every sample 

 studied, but sometimes hours of patient search were required before 

 such a single individual was found. 



* Mansfield, G. R., Econ. Geol., vol. 35, pp. 547-566, 1920, U. S. Geol. Survey Bull. 527, 

 1922. 



B Nelson, Wilbur A., Geol. Soc. America, Bull., vol. 33, No. 3, pp. 605-615. 



