90 GEOLOGY OI' Tin: IJLACK II ILLS. - 



Glaucoiiite or ^nvensaiid is a liydrous silicate of protoxide of iron 

 and i»otasli, witli a variablo 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 

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

 liicketts. 



12 3 



Silica 50.70 50.70 54.10 



Aliimiiui 8.03 10.80 10.02 



Iron i)rot().\i(le 22.50 8.G0 22.98 



I'otash 5.80 8.20 C.75 



Ma-,nicsia .. 2.1G 3.70 0.82 



Lime Lll 1.3G 



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' grou])s of strata. Such strata are 

 widely recognized in P^urope, 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 

 ]3ailey filling the recent shells of Pohjtlialmia (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. PealeJ in the middle canon of the Madison River; and Dr. 

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



• S.vstem of Mineralogy. Dana. p. 4(K. 



t Aiiioricau .Idurn.-il of Science. 2, xxiv, p. '281. 



{ Gcohiyical Survey of the Territories, 1872. p. IGO. 



$ Ihid., p. 72. 



