THE CRETACEOUS PERIOD 



419 



Farther south in Alabama and Mississippi, the Cretaceous 

 system includes several hundred feet of chalk or soft lime- 

 stone. In the belt where this chalky formation outcrops, the 

 soil is so good that the cotton plantations there are the 

 richest in the South. In the same region the slave popula- 

 tion was densest before the Civil War and the proportion of 

 negroes is now largest. 



An interior sea divides North America. Very early in 

 the Cretaceous period the sea which lay south of the United 

 States began a slow advance northward over the nearly 



FIG. 438. Stereogram of part of the Great Plains early in the Cretaceous 

 period, showing the probable relations of the zones of different kinds of 

 sediments. 



level surface of the land. Along the shores of the sea there 

 were probably marshes and lagoons such as now fringe the 

 low coast of Texas. Still farther inland, there were broad 

 plains over which the sluggish rivers were spreading fine 

 sediments (Fig. 438). 



As the sea advanced northward, the shore conditions must 

 have migrated slowly in front of it, the streams constantly 

 depositing sediment farther and farther north as the advance 

 continued. These deposits of the alluvial plains appear to 

 be represented in the oldest Cretaceous formation of the 

 Great Plains region, the Dakota sandstone. In this sand- 

 stone many leaves of land plants have been found (Fig. 439). 

 As the leaves are unworn, and are well preserved, one can 

 hardly suppose that they were washed out to sea and 



