proceedings: geological society 197 



the more abundant minerals were determined in each size by immer- 

 sion in oils of known index of refraction. In order that a comparison 

 with bentonite might be made, seventeen specimens of sediment from 

 associated beds were also sized. 



Bentonite contains two classes of material. On the one hand, the 

 sands and silt, which include grains that range from 0.005 to 1.0 mm. in 

 diameter, are largely plagioclase (andesine), orthoclase, and biotite, 

 with accessory quartz, glass, apatite, zircon, and agate. The grains of 

 plagioclase and orthoclase are highly angular and are uniformly fresh. 

 Most of the grains of quartz are also angular, but a few are well rounded, 

 whereas the grains of apatite and zircon are fresh terminated crystals. 

 On the other hand, the clay, which includes all grains less than 0.005 

 mm., shows by analysis water, silica, and alumina. The molecular ra- 

 tio of silica to alumina is about 9 to 1, and therefore much higher than 

 that in any of the common hydrous silicates of alumina. The dried 

 powder is birefracting (biaxial negative) and the average refringence of 

 different specimens ranges from 1.52 to 1.59. After the clay has been 

 immersed in water, however, it is isotropic and therefore amorphous. 

 This clay forms 73 to 86 per cent of the beds of pure bentonite, whereas 

 the sands and silts make up the remainder. In the beds of impure 

 bentonite however, the proportion of clay is lower and ranges from 54 

 to 61 per cent. One of the peculiar properties of the clay is its tend- 

 ency to swell to six or eight times the original volume when immersed 

 in water. 



The clays, shales, and sandstones of the adjacent sediments which 

 have been examined, are common types of quartzose sediments. Sub- 

 angular grains of quartz and chert make up a large part of the coarsest 

 sizes of most of the sediments, but a little orthoclase, biotite, plagio- 

 clase, and chlorite are commonly present. Quartz also forms an ap- 

 preciable part of the finest silt and clay portions, but the birefracting 

 clay of bentonite was not noted in any of the clay portions of these sedi- 

 ments. 



In contrast with adjacent sediments, therefore, bentonite contains 

 little if any quartz, but fresh plagioclase, orthoclase, and biotite pre- 

 dominate, and glass is locally present. The comparison of the mineral 

 constituents of bentonite with volcanic ash from several localities, shows 

 that whereas the variety and proportions of the minerals are strikingly 

 similar, the proportion of glass grains in volcanic ash is represented in 

 bentonite by the highly siliceous clay just described. The resemblance 

 in mineralogy and chemical composition of the two classes of materials 

 warrants the conclusion that bentonite is volcanic ash in which the 

 glass has been hydrated and has lost most of its constituents except 

 silica and alumina. An inquiry into the stratigraphic relations of the 

 beds of bentonite appears to show that this alteration must have taken 

 place before the ash was laid down. In the case of the Eocene clays, 

 which are flood-plain deposits, the decomposition took place after depo- 

 sition. 



