56 ■ THE EOCEXE DEPOSITS OF MAEYLAXD 



derived materials are deposited in perceptible yet small amounts. The 

 production of glauconite seldom reaches to greater depths than 900 

 fathoms, and most commonly takes place between 100 and 200 fathoms. 

 The entrance of large rivers into the sea or the prevalence of strong 

 currents bearing sediment tends to interfere with its formation, so that 

 its area of distribution is seldom continuous for great distances. 



Although glauconite is not known to be formed except in the pres- 

 ence of land-derived materials, its production is accomplished through 

 the intervention of foraminifera. Their connection with the formation 

 of glauconite was first shown by Ehrenberg' in 1855, as the result of a 

 study of greensand from many deposits in Europe and America. Pro- 

 fessor Bailey ' in the succeeding year stated that the formation of green- 

 sand is likewise taking place on the floor of existing seas and probably 

 under the same conditions that existed in past geological time. 



According to Murray and Eenard, the chambers become filled with 

 muddy sediment, and " if we admit that the organic matter inclosed in 

 the shell, and in the mud itself, transforms the iron in the mud into 

 sulphide, which may be oxidized into hydrate, sulphur being at the 

 same time liberated, this sulphur would become oxidized into sulphuric 

 acid, which would decompose the fine clay, setting free colloid silica, 

 alumina being removed in solution; thus we have colloid silica and hy- 

 drated oxide of iron in a state most suitable for their combination." 

 The potash which is necessary to complete the composition of glauco- 

 nite is regarded as derived from the decomposition of the fragments of 

 crystalline rocks or their common constituents, orthoclase and white 

 mica. 



Two conditions, then, are requisite for the formation of glauconite: 

 First, the deposition of mineral particles of land-derived origin; and 

 second, the presence of foraminifera. In the absence of either, glauco- 

 nite will not be produced. On the other hand, it is retarded, and finally 

 ceases altogether, as the amount of deposition of land-derived materials 

 increases adjacent to. the coasts. Only, then, within circumscribed 

 liuiits, which are constantly subject to modification, is the formation of 

 glauconite possible. 



1 Abliandl. K. Akad. Wiss. zu Berlin, 18.55, pp. 8.5-176. 

 ■-'Froc. Boston Soc. Nat. Hi&t., vol. v, 1856, pp. S64-36S. 



