370 HISTORICAL GEOLOGY 



The rocks, as we now find them, are thick clastic formations, 

 usually called the Pocono sandstone below, and the Mauch 

 Chunk shale above. Ripple marks and sun cracks in the 

 shales indicate that they were deposited in shallow water; 

 and a close study of them has recently made it fairly certain 

 that they represent a great flat delta plain over which rivers 

 in a semiarid climate spread silts and sands in times of 

 flood. Occasional coal seams tell of the existence of marshes 

 upon the surface of this delta plain. 



Limestone in the central and western states. As we 

 trace them farther west and south, the land-derived sediments 

 become finer, and limestone increases in prominence. From 

 Indiana westward massive limestones form the bulk of the 

 Mississippian system. The same formation reappears in the 

 Black Hills, parts of the Rocky Mountains, and the Arizona 

 plateaus, and is believed to underlie nearly all of the Great 

 Plains. This extensive limestone series implies a clear open 

 sea remote from rugged lands. That its genial waters 

 abounded in animals of the sea is proved by the crinoids, 

 corals, and other fossils with which the strata are locally 

 crowded. This is true especially in the Mississippi and Ohio 

 Basins, or, in other words, near the border line between the 

 muddy and the limy bottoms. The deep sea explorations of 

 the " Challenger Expedition " some years ago brought out 

 the fact that animals are always extraordinarily abundant 

 near the mud line or the outer edge of the muddy area ; there 

 the conditions of life seem to be more favorable than else- 

 where. 



Sedimentation outside of the interior sea. A body of 

 water covering the southern peninsula of Michigan was at 

 this time more or less isolated from the great interior sea. 

 The strata which accumulated there are associated with salt 

 and gypsum, suggesting that the local climate was not moist, 

 and that the basin was cut off from direct connection with 

 the sea. 



Again, in Nova Scotia sediments were laid down in basins 



