DESALIXIFICATIOX: CONCRETIONS 763 



oxide and a corresponding decrease in calcium carbonate. It is 

 true that these deposits have been regarded as formed directly in 

 lagoons and cut-offs (Smyth-29) along the sea coast of the time, 

 the iron being brought by the wash from the crystalline old land. 

 This theory has, however, been discarded by some recent students of 

 the subject in favor of the older replacement theory. 



The replacement of calcareous bodies of organic origin by iron 

 pyrites and other mineral substances will be more fully discussed 

 in a later chapter. 



V. Desalinification. 



Among other diagenetic processes of importance may be men- 

 tioned the desalinification of old marine sediments. As already 

 noted, the amount of salt absorbed by marine sediments varies 

 greatly, chiefiy in proportion to their pore space. (Gerbing-15 : ^5, 

 118.) After these sediments are raised into the zone of circulating 

 ground water, a slow removal of these salts takes place. Under 

 arid climatic conditions, as already noted, this may go on more 

 rapidly, and the leached salt may be redeposited in salinas and 

 desert salt basins. 



VI. Formation of Concretions. 



This has already been discussed at some length in a previous 

 chapter (see pp. 718-720) and need be dwelt on only briefly here. 

 Percolating waters carrying lime, silica or other substances in solu- 

 tion will deposit these in the strata at favorable localities, forming 

 concretions of lime, of clay-iron-stones, or of silica. The first are 

 common in calcareous shales, often growing to large size, with a 

 corresponding deformation of the enclosing layers, through the 

 pressure of the growing concretion. Not infrequently the concre- 

 tions become confluent, forming a concretionary limestone bed. 

 The nucleus of the concretion is very often some organic fragment 

 or a shell. In the Champlain clays of Cumberland, C^ntario, entire 

 specimens of fish are common. Fish remains are found in similar 

 concretions of the glacial and post-glacial clays of Norway, 16 

 species having been recognized so far. 



The common types of concretion in the jMesozoic and Palaeozoic 

 shales are the septaria already described. These are often of great 

 size, examples 10 feet in diameter occurring in the Devonic of New 

 York. They not infrequently contain a fish bone as a nucleus, 



