190 



STRATIFIED OR SEDIMENTARY ROCKS. 



Fig. 171. — A Curious Form of Concretion (after Gratacap), 



from that of a pea to six and eight feet in diameter. Often, however, 

 instead of the spherical form, they take on various and strange and 

 fantastic shapes (Fig. 171), sometimes like a dumb-bell, sometimes a 

 flattened disk, sometimes a ring, sometimes a flattened ellipsoid, regu- 

 larly seamed on the surface like the shell of a turtle (turtle-stones). 

 They are often mistaken by unscientific observers for fossils. 



Kinds of Nodules found in Different Strata.— In sandstone strata the 

 nodules are commonly carbonate of lime or oxide of iron (lime or iron 



balls). In clay 

 strata they are 

 carbonate of lime 

 or carbonate of 

 iron (clay iron- 

 stone of coal 

 strata), or a mix- 

 ture of these 

 (Roman cement 

 nodules of the 

 London clay). 



In limestone the nodules are always silica, and conversely silica 

 nodules are peculiar to limestone. The flint nodules of the chalk are 

 remarkable for being arranged in planes parallel to the planes of strati- 

 fication (Fig. 172). 

 Sometimes the sili- 

 matter segre- 

 in continuous 

 strata of siliceous 

 limestone (Fig. 173). 



In the cases thus 

 far spoken of, the nod- 

 ules are scattered 

 through the mass of 

 the strata or arranged 

 in planes parallel to 

 the planes of stratifi- 

 cation. But in SOme FlG " ^-Chalk-Cliffs with Flint Nodules. 



cases the whole mass of the rock assumes a concretionary or concentric 

 structure. The cause of this is still more difficult to explain. 



ceous 



Fossils : theik Origin and Distribution. 



Stratified rocks, as we have already seen, are sediments accumulated 

 in ancient seas, lakes, deltas, etc., and consolidated by time. As now, 

 so then, dead shells were imbedded in shore-deposits ; leaves and logs of 

 high land-plants, and bones of land-animals, were drifted into swamps 



