720 PRINCIPLES OF STRATIGRAPHY 



matter, chiefly calcite, the accumulation of which helps to widen 

 and extend these internal fissures. Concretions of this type are 

 familiarly known as septaria. from the fact that when they have 

 been worn on the surface the veins sometimes weather in relief, 

 _ and thus produce a septate structure. Often, however, the veins are 

 depressed. These concretions are sometimes thought to be petri- 

 fied turtles, and in many localities are known by the name of 

 "turtle stones." They are abundant in the Devonic shales of 

 eastern North America and in the Cretacic beds of the interior, 

 while the Jurassic of western Europe is famous for its remarkably 

 beautiful examples. 



Excretions are centripetal concretions "consolidation progress- 

 ing inward from the exterior." (Dana.-Manual, 4th ed. — q8.) 

 They are represented by nodular shells of sand cemented by iron 

 oxide, and generally filled by more or less unconsolidated sand or 

 contain other shells of cemented sand. Todd holds that normal ac- 

 cretions or intercretions of ferrous carbonate, on coming in contact 

 with waters charged with carbon dioxide and oxygen, will begin to 

 dissolve and a shell of ferric hydroxide will form on its surface by 

 precipitation. The iron carbonate of the portion of the concretion 

 within this shell will similarly be dissolved and reprecipitated as 

 ferric hydrate, a second shell thus being added on the interior of the 

 first. The impurities of sand or clay in the original concretion re- 

 main behind, in a loose condition. Thus while the actual thickening 

 of the shell is on the inside, and the growth, therefore, from without 

 inward, the molecular movement of the iron salts is from within 

 outward, i. c, from the core of the original concretion to the inner 

 wall of the shell. When the process is completed a loose mass of 

 sand or other impurities alone remains behind, or, if the original con- 

 cretion was free from impurities or nearly so, the resultant excre- 

 tion may be hollow. Excretions formed within ferruginous sand- 

 stones are often the cause of puzzling hollow cavities. 



Incretions. These are cylindrical concretions with a hollow 

 core. Todd infers that these originate where the walls of a cavity, 

 like the cylindrical tube left by a decayed root in the sand or clay, 

 serve as the nucleus for deposition. The iron is drawn from the 

 surrounding material and moves inward to the center, where it 

 is added to the central cylinder, which grows in thickness outward 

 by the addition of successive shells on the exterior, until the sur- 

 rounding matrix is depleted. 



Minute concretions of this type are common in the loess. They 

 consist of carbonate of lime, and resemble clay pipe stems in size 

 and form. 



