262 STUDIES FOR STUDENTS 



and fragmentary sections such a comprehensive knowledge of the 

 limits and surface nature of the original formation becomes, however, 

 a difficult problem. Certain rules should, therefore, be formulated, 

 by following which the local geographic conditions may be most 

 largely eliminated from the problem of the climatic interpretation. 

 Such rules may be stated as follows: First, the general direction of 

 the former land on the one hand and of the sea on the other is ordi- 

 narily readily determinable. The deposit is coarsest, the slope of the 

 river-plain steepest, the surface and underground drainage best, over 

 those portions of the deposit nearest the source of sediment. The 

 highest proportion of continental as opposed to marine strata will 

 ordinarily be found in the same region. This, therefore, will be the 

 most favorable place for the study of all but the chemical or organic 

 deposits. Second, toward the landward or upstream side the thick- 

 nesses of the formation will commonly vary along successive outcrops. 

 Such variable thicknesses may be due either to excessive subsidence 

 causing drainage of the sediments toward and into the basin, filling it 

 in ; or excessive sedimentation building up piedmont slopes, the excess 

 passing outward in other directions, or to a combination of both 

 conditions. Observation of ancient geosynclines, such as that which 

 faced Paleozoic Appalachia, shows that in many cases excessive sedi- 

 mentation in the vicinity of some large river appears to be the more 

 common and fundamental cause, the zone of maximum deposition 

 being characterized at the same time by the coarsest material and a 

 decreased proportion of chemical and organic deposits. In continen- 

 tal formations, therefore, the region of maximum thickness, as well 

 as greatest coarseness, is usually the most favorable for the study of 

 the mechanical conditions of deposition. Third, the regions of 

 scanty sedimentation and the paludal zone facing the ancient water 

 body are the most favorable for the development of chemical and 

 organic deposits. Under arid climates will here be found beds of 

 salt and gypsum intercalated with both continental and marine 

 argillaceous strata, possibly associated with a small amount of varie- 

 gated shales; but, judging from geological experience, never deposits 

 of carbon. Under typically rainy climates, on the other hand, what- 

 ever occasional deposits of salt and gypsum may form are speedily 

 washed away during a following season of rain, and the permanently 



