502 PROCeSDINGS: GEOIvOGICAL SOCIETY 



ferric over ferrous iron and the fact that glauconite seems to be an 

 orthosihcate points in the same direction. Then too, the molecule of 

 water which appears to be an essential part of its composition would 

 place it rather with the vermiculites than the chlorites. Allying it 

 with the chlorites, on the other hand, is the magnesia content which 

 seems to be pretty persistent. The chemical composition presents a 

 fundamental difficulty, however, since it is uncertain whether the 

 mineral is a definite crystalline substance or a colloform mixture. 



Glauconite is a characteristic phase of certain terrigenous marine 

 sediments and very widely distributed around all the continents. 

 But it is rare in the delta type of terrigenous deposit on the one hand 

 and in pellagic deposits on the other. Concerning its mode of forma- 

 tion there is great uncertainty, but it seems to develop in small segre- 

 gated pellets of clay (generally i mm. or less diam.) when there is 

 organic matter present. The process assumed has been the reduction 

 of the sulfates of sea water to sulfides which form iron sulfide with 

 the iron present in the clay. This sulfide is believed to combine with 

 silica in the clay and the iron silicate thus formed to take up potassium 

 from the sea-water. The combination of iron sulfide with silica to 

 form a silicate has been brought about experimentally, but the reaction 

 was prevented by an excess of hydrogen sulfide. On the basis of the 

 facts summarized the speaker presented the hypothesis that glauconite 

 is one of three characteristic modes of occurrence of iron in marine 

 deposits, being an intermediate product between high organic content 

 and reducing conditions yielding the sulfide on the one hand, and low 

 organic content with oxidizing conditions producing the hydrous oxides 

 on the other. 



Geologically, also, glauconite is a very widely distributed and com- 

 mon mineral, having been found in deposits of every period. It occurs 

 particularly characteristically, however, just above unconformities, 

 that is in transgressing (perhaps also in regressing) formations, and 

 preeminently in connection with the great transgression of the lower 

 part of the Upper Cretaceous. This stratigraphic position has not 

 been adequately explained. The frequent association of glauconite 

 and phosphate in both recent and ancient deposits is familiar, and 

 Murray has pointed out a relation, at least partial, of phosphate with 

 great extremes of surface temperatures of the ocean. Cayeux has 

 suggested that such instabilities of temperature might be produced by 

 extensive movements of the earth's crust and the same explanation 

 may be applicable, in part, to the geologic occurrence of glauconite. 



G. R. Mansfield: General features of the glauconite marls of New 

 Jersey. 



(This paper is to be published in Economic Geology.) 



E. O. Ulrich: Paleozoic glauconite zones and suggestions as to their 

 origin. R. W. StonE, Secretary. 



