230 



DISPLACEMENTS OE STRATA 



plained. The material of which they are made must have been 

 scattered through the stratum and then gathered together at a 

 later period. Such nodules have been observed in the process of 

 formation in modern sediments, and it has further been noticed 

 that when finely powdered substances are mixed together, certain 

 of them do segregate into lumps. These observations, however, 

 merely confirm the conclusion that concretions are due to segre- 

 gation of scattered material in the stratum, they give us no 

 explanation of the fact. 



Fig 



Ironstone concretion, split open to show the fossil leaf which forms the 

 nucleus. Mazon Creek, Illinois. 



The commonest concretions are those of clay in various kinds 

 of rock, of flint and chert in limestone, and of ironstone in clay 

 rocks. 



Displacements of Stratified Rocks 



It is evident that the stratified rocks which form the land must 

 have been changed, at least relatively, from the position which 

 they originally occupied, since the great bulk of them were laid 

 down under the sea. Originally they must have been nearly 

 horizontal, for this is a necessary result of the operation of gravity. 

 Just as a deep fall of snow, when not drifted by the wind, gradually 



