PALEOBOTANY 583 



rocks occur in layers/ the deeper lying layers (with ex- 

 ceptions readily explained by geologists) being older than 

 those above, or nearer the surface. Moreover, as a result 

 of a second submersion following elevation and erosion, 

 subsequent layers were often deposited with an uncon- 

 formity on the weathered and eroded surface under- 

 neath. 



By the presence of fossil imprints of rain drops, foot- 

 prints, ripple marks, and mud cracks, and by the character 

 of the plant and animal fossils which they contain, we 

 know that most sedimentary rocks were deposited in 

 shallow water, not far from the shore line. But since 

 these same rocks may have a thickness of thousands of 

 feet we know the area of sedimentation must have been 

 slowly sinking while the sediment was being deposited. 

 As a result of the enormous pressure of the overlying 

 material, of the deposit of cementing substances from 

 solution, and of other causes, the sedimentary deposits 

 became, in time, converted into solid rock. 



508. Classification of Rock Strata.^By a study of the 

 fossils which the rocks contain, geologists have been able 

 to classify the various strata according to their age. 

 As a result of the period of erosion, indicated by un- 

 conformity, the transition from the stratum of one age 

 to that of another is often abrupt, the fossils in successive 

 periods being quite characteristic of the given stratum 

 or period. In other cases, as for example between the 

 Silurian and Devonian in New York State, there is no 

 unconformity, and this renders it more difficult to decide 

 just where the plane of division lies. The names and 

 order of occurrence of the known rock strata are given in 



' Several layers form a stratum or bed. • 



