270 STUDIES FOR STUDENTS 



matter indefinitely accumulates, and the iron is eliminated or at least 

 reduced partly to the ferrous conditions, giving rise to the blue and 

 green clays found in the subsoils of certain undrained lands. From 

 the discussion on the climatic significance of color, as given in a later 

 portion, it is believed that the usual yellow or brown of such flood-plain 

 deposits frequently deepens upon the consolidation into shades of 

 red or deeper brown. When such flood-plain deposits are buried 

 and lithified the upstream portions will consequently be found some- 

 what more arenaceous, varying from red to brown sandstones and 

 usually inclosing red, green, and some black shales ; the last in very 

 subordinate quantity. Over the terminal land portions of the deposit 

 on the contrary the sandstones should be finer grained and the quantity 

 of shales should increase. With this increase in shales, grey, green, and 

 black varieties should be relatively more abundant and thin lenticular 

 discontinuous coal beds may be expected to occur. Thick, uniform, 

 and widespreading coal deposits are, however, theoretically impos- 

 sible, since the swamp areas are restricted to the lowest-lying portions 

 of the plain. 



Organic characteristics. — k forest growth normally covers the 

 entire surface of such flood plains, varying from mesophytic to hydro- 

 phytic types, salt marshes marginal to the sea and internal shallow 

 lakes alone being occupied by reedy growths. The alternate wetting 

 and drying occurring in such soils leads to the rapid humification of 

 animal and vegetable substances, and ultimately to their complete 

 destroyal. Consequently, but few fossil evidences will remain 

 beyond the casts of leaves and trunks occurring in the lighter-colored 

 shales and sandstones, and the preservation of some carbonized tissues 

 in the occasional black and coaly shales. 



Conclusion on unappreciated extent of such deposits. — In conclusion 

 it may be said that the deposits of intermittently rainy climates are 

 of a chemical and organic character intermediate between those of 

 continually rainy climates on the one hand and those of a subarid or 

 arid character on the other, lacking the sharply distinguishing char- 

 acteristics of each. In consequence of the absence of such distinctive 

 marks such deposits, while abundant, may be the most diflicult of con- 

 tinental formations to distinguish and convincingly separate from 

 those of shallow-water, off-shore marine origin. 



