670 PRINCIPLES OF STRATIGRAPHY 



abundance of shells and other remains of marine organisms. Since 

 they are formed opposite the mouths of great rivers, which may 

 sweep terrestrial organisms into the sea, such remains may also be 

 expected in these deposits. 



The Green Mud. This ranges in depth from 180 to 2,300 meters 

 (99 to 1,250 -(- fathoms), and may contain as much as 56 per cent, 

 of lime. In shallower depths this green mud passes into greensands. 

 The percentage of clay in the green mud varies from 24 to 48. 

 Glauconite is one of the chief constituents of both the greensands 

 and green muds. 



Greensand. This is essentially the mineral glauconite, an impure 

 hydrous silicate of iron and potassium. Clarke states that, "accord- 

 ing to the best analyses, glauconite probably has when pure the 

 composition represented by the formula Fe'" KSioO,; aq., in which 

 some iron is replaced by aluminum, and other bases partly replace 

 K." This is forming in the present ocean near the "mud line" 

 around continental shores. It is never pure, but contaminated by 

 alteration products and other substances, and hence its composition 

 varies widely. In the present ocean it is chiefly formed on the in- 

 terior of foraminiferal shells, and it is believed that the decompos- 

 ing organic matter in these shells is responsible for its formation. 

 According to Murray and Renard (62:j5'j), the shell is partly 

 filled with fine silt or mud, and upon this the organic matter will 

 act. "Through intervention of the sulphates contained in the sea 

 water, the iron of the mud is converted into sulphide, which oxi- 

 dizes later to ferric hydroxide. At the same time alumina is re- 

 moved from the sediments by solution, and colloidal silica is lib- 

 erated. The latter reacts upon the ferric hydroxide in presence of 

 potassium salts extracted from adjacent minerals, and so glauconite 

 is produced." (Clarke-2ob i^oi".) The constant association of the 

 glauconite shells with the debris of rocks in which potassium-bear- 

 ing minerals such as orthoclase and muscovite occur seems to sus- 

 tain this view of Murray and Renard of the origin of glauconite. 



Glauconite may, however, form under other conditions than 

 those now obtaining in the glauconite region of the present oceans. 

 L. Cayeux (15) has shown that in certain instances glauconite has 

 formed subsequent to the consolidation of the rocks in which it 

 occurs. Cayeux shows that this mineral can also form without the 

 intervention of organisms. 



While glauconite of the present sea and in the sedimentary rocks 

 is crystalline (monoclinic. La Croix), having a definite cleavage, 

 though not crystal form, the mineral caladonite of nearly identical 

 composition is earthy in texture and never granular. This is a de- 



