96 GEOLOGY OF THE BLACK HILLS. 



Glauconite or greensancl is a hydrous silicate of protoxide of iron 

 and potash, with a variable amount of alumina, but "the material is 

 mostly, if not always, a mixture, and consequently varies much in com- 

 position."* 



The following analyses will indicate the composition of the min- 

 eral : 1 is from the Greensand of New Jersey ; 2 from the Lower Silu- 

 rian of the Island of Orleans, Canada ; and 3 from the Potsdam of the 

 Black Hills. The first two are by Dr. T. S. Hunt, the last by Dr. P. de P. 

 Ricketts. 



12 3 



Silica 50.70 50.70 54.10 



Alumina 8.03 10.80 10.92 



Iron protoxide 22.50 8.60 22.98 



Potash.. ... 5.80 8.20 6.75 



Magnesia 2.16 3.70 0.82 



Lime 1.11 1.36 



Soda 0.75 0.50 3.02 



Water 8.95 8.50 0.04 



100.00 100.00 99.99 



Greensand has been particularly observed in the Cretaceous, and cer- 

 tain beds of the formation sometimes contain 70 to 90 per cent. This 

 feature of the Cretaceous is so persistent, that it has caused the application 

 of the name "Greensand" to certain groups of strata. Such strata are 

 widely recognized in Europe, and in the United States are well developed 

 in New Jersey, Alabama, and Massachusetts (Gay Head). The mineral 

 abounds also in the Tertiary rocks, is recognized in various members of 

 the Silurian system in this country and Europe, and has been found by 

 Bailey filling the recent shells of Polytlialmia (foraminifera) from deep-sea 

 soundings on the Atlantic coast.f 



In the Potsdam of the Rocky Mountains greensand has been found by 

 Dr. A. C. Pealef in the middle canon of the Madison River ; and Dr. 

 Hayden,§ near the mouth of the Gallatin, finds in the Potsdam "layers of 



* System of Mineralogy. Dana. p. 462. 



t American Journal of Science. 2, xxiv, p. 281. 



| Geological Survey of the Territories, 1872. p. 1C0. 



§ Ibid., p. 72. 



