CLASSIFICATION GRADATIONAL AND LITHOLOGTC CRITERIA 467 



of the evidence cited, also the fact that the criteria are as clearly de- 

 veloped in cases indicating a relatively brief existence of land conditions 

 as in those of much longer duration, it seems to me we are justified in 

 assuming actual sea withdrawal and consequent increasing emergence in 

 practically all localities where imperfections of the marine sedimentary 

 record are indicated by faunal and stratigraphic criteria. 



Gradational and lithological Criteria 

 general discussion 



The leveling processes of nature are ever at work. The high places of 

 the earth's surface are cut down, the low places filled up. Given lands of 

 high relief under average conditions, the erosional agents work not only 

 faster but also less finely than on lands of low relief. In the former the 

 surface rocks are rapidly broken up and carried to the depressions without 

 much decay, in the latter, slow decomposition in situ and slower re- 

 moval of the regolith or rock mantle prevails; and under conditions of 

 approximate baselevel erosion becomes practically negligible. We have 

 then in the composition, degree of fineness, and amount of the clastic 

 matter in a formation fairly definite clues respecting the altitude, the 

 kinds of rock exposed, and the climate prevailing on the land from which 

 the material was brought. Thus limestones, especially when it can be 

 shown that calcareous deposition occurred at the shores as well as farther 

 out in the seas, suggest low land. Shales also, whether calcareous, clayey, 

 or siliceous, usually indicate gentle gradients and comparatively low 

 lands. Fine-grained red deposits commonly indicate submergence of such 

 relatively low lands after a long period of emergence. When the red 

 sediments are coarser, or when they include beds of salt or gypsum, these 

 facts argue for arid climates, but throw little light on the matter of relief 

 of the contributing lands. 



If the material is relatively coarse and was transported by water, we 

 infer, or I might say we laiow, that the contributing land was high or 

 had locally steep gradients. If this coarse material rests on a limestone 

 formation, we are justified in saying that elevation of near-by land areas 

 occurred between the two kinds of deposits and perhaps continued into 

 the time occupied by the deposition of the clastic formation. This 

 elevation may be but a local affair; but if it can be established through 

 the aid of fossils that elevation is similarly indicated in many places — 

 perhaps all over the world — then we conclude that we are dealing with 

 the local aspect of a phenomenon of extraordinary taxonomic importance. 

 It is not essential that vulcanism or strong folding or decidely angular 



