PALEOBOTANY 583 
rocks occur in layers, 1 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 
1 Several layers form a stratum or bed. 
