220 



THE STRUCTURE OF STRATIFIED ROCKS 



Usually, a section of thick rock masses shows continual change 

 of material at different levels. Figure 77 is a section of the rocks 

 in Beaver County, Pennsylvania, in which several different kinds of 

 beds register the changes in the physical geography of that area. 

 At the bottom of the section is a coal seam (No. 1), the con- 

 solidated and carbonized vegetable matter which accumulated 

 in an ancient fresh- water swamp. Next came a subsidence of 

 the swamp, allowing water to flow in, in which were laid down 

 shales of different kinds (No. 2). This 

 fine-grained clay rock, in this particular in- 

 stance, probably represents quiet, sheltered 

 water, rather than any considerable depth 

 of it. The accumulations of mud eventually 

 shoaled the water and enabled a second 

 peat swamp to establish itself; this is regis- 

 tered in the second coal bed (No. 3), the 

 thinness of which indicates that the second 

 swamp did not last so long as the first. 

 Renewed subsidence again restored marine 

 conditions, as is shown by the layer of cri- 

 noidal limestone (No. 4) which overlies the 

 second coal bed. This depression produced 

 a greater depth of water, and the distance 

 from land was sufficient to prevent the influx 

 of terrigenous sediment. Next, the water 

 was shoaled by an upheaval, and argilla- 

 ceous sands were laid down, which now form 

 the flaggy sandstones (No. 5) overlying the limestone. The twenty- 

 five feet of sandstone, aided by the continued slow rise of the sea- 

 bottom, silted up the water and allowed a third peat bog to grow, 

 the result of which is the third coal seam (No. 6), while a repeti- 

 tion of the subsidence once more brought in the water, in which 

 were laid down the seventy feet of gravel at the top of the sec- 

 tion. In this fashion the succession of strata records the changes 

 of land and sea which were in progress while those strata were 

 forming. 



Fig. 77. — Section in coal 

 measures of western Penn- 

 sylvania. (White.) 



