GREENSANDS 671 



composition product of augite in various basaltic rocks, and may be 

 identical with glauconite (Clarke). Another mineral of similar 

 appearance is greenalite, found in the iron-bearing rocks of the 

 Mesabi range of Minnesota. It is free from potassium, the iron 

 being practically all in the ferrous state (Leith-56 : J-^o), while in 

 glauconite, where potassium is an essential constituent, the iron is 

 mainly in the ferric state. Cayeux has also observed that glau- 

 conite is frequently present in arable soils, in all conditions from 

 perfect freshness to complete alteration into limonite, to which 

 Clarke remarks that the formation of the species is perhaps "one 

 of the modes by which potassium is withdrawn from its solution in 

 the ground water." (20b :^9^.) "Probably, in all their occur- 

 rences," says Clarke, "the final reaction is the same, namely, the 

 absorption of potassium and soluble silica by colloidal ferric hy- 

 droxide. In the ocean these materials are prepared by the action of 

 decaying animal matter upon ferruginous clays and fragments of 

 potassium-bearing silicates. In the sedimentary rocks, where glau- 

 conite appears as a late product, the action of percolating waters 

 upon the hydroxide would account for its formation. In igneous 

 rocks the hydroxide is derived from augite, or perhaps from olivine,, 

 and percolating waters again come into play. Thus the various pro- 

 ductions of glauconite and caladonite become the results of a single 

 process, which is exactly equivalent to that in which potassium com- 

 pounds are taken up by clays." (20b :^p^.) 



Greensands are found in nearly all formations from the Cambric 

 to the present, and often constitute a predominant element of the 

 formation, as in the Cretacic Greensands of England and America. 

 Analyses of the mineral from different horizons show on the whole 

 a close correspondence in composition, there being, however, a stead- 

 ily decreasing percentage of potash from the older to the more re- 

 cent. In the table on page 672, the average composition of various 

 glauconites is given : (a) from the Lower Ordovicic of Minnesota, 



(b) from the Cretacic Greensands of Wooburn, Antrim, Ireland, 



(c) from the Cretacic Greensand marls of Hanover County, Vir- 

 ginia, (d) modern oceanic glauconite, mean of four analyses from 

 Challenger dredgings (analyses b, c, d quoted from Clarke-2ob : 



494)- 



The mechanical composition of typical greensand marls (Nave- 

 sink) of the Cretacic of New Jersey has been determined by 

 Prather (73: 162; 74) to be as follows: 



Fine clay (settling out of suspension in water in the course of 

 from I to 24 hours), 14.41 per cent. ; sand, almost wholly composed 

 of pure glauconite, pyrite, shell fragments and Foraminifera (set- 



