(iy^ 



PRINCIPLES OF STRATIGRAPHY 



Table of Analyses of Glauconite from Various Horizons. 



Si02 



AI.O3 



FesOs 



FeO 



MnO 



MgO 



CaO 



NaaO 



K2O 



H2O 



Total . 



(a) 

 Ordovi- 



48.18 

 6.97 



27.08 



125 

 7.40 



8.75 



99 63 



(b) 

 Cretacic 

 Ireland 



40.00 

 13.00 

 16.81 

 10. 17 



1.97 

 1.97 

 2. 16 

 8.21 

 6. 19 



100.48 



(c) _ 

 Cretacic 

 Virginia 



5156 



6.62 



15.16 



8.33 



0.95 

 0.62 

 1.84 



415 

 10.32 



99-55 



(d) 

 Modern 



53-61 

 956 



21 .46 

 1-58 



trace 

 2.87 



1-39 

 0.42 



3-49 

 596 



100.34 



tling out of suspension in water in five minutes or less), 85.59 per 

 cent. A sample from a lower bed of the same formation (Nave- 

 sink) gave clay as above 8.18 per cent.; sand, composed about 

 equally of pure glauconite and of rounded grains of quartz, with 

 some mica, 91.82 per cent. 



Many grains of glauconite represent internal molds of forami- 

 niferal shells, though frequently further enlargement of these molds, 

 after the solution of the shell, produces irregular glauconite nodules. 

 W. B. Clark (17: 2^8; 18) believes that the New Jersey glauconites 

 were formed in much the same manner as the modern glauconites 

 of the ocean, but Prather (74:509) points out some objections to 

 this interpretation, and cites evidence of shallow-water conditions 

 during the accumulation of these glauconite sands. He finds that 

 the casts of Foraminifera are the exception. The greensand grains 

 of the Lower Ordovicic dolomites of Minnesota are regarded by 

 Hall and Sardeson (42: 186) as having their origin "in the chem- 

 ical conditions of the mingled mineral matters of the including 

 rocks. . . ." and not in the chemical changes within foraminiferal 

 shells. (See Htmt-49: joj; 50: ig6, ^09; 51 : 2fj/.) Though mod- 

 ern glauconite deposits are chiefly confined to depths at or for some 

 distance below the 100- fathom line, the frequent association of this 

 mineral with typical littoral deposits, and in formations containing 

 a littoral fauna, indicates that in the past it may have formed in 



