EVALUATION OF STRATIGKAl'll !C CRITERIA 415 



Beyond the initial problem of the separation of the subaerial from the 

 subaqueous portions, however, many subsidiary problems arise. The very 

 complexity of chemical composition, of structure, and of fossil content 

 make possible by their variations and combinations a highly significant 

 geologi(t record. The stratigraphic characters of land deposits especially 

 bear the impress of climate and topography. The subaqueous deposits 

 are related in their nature to life, depth of water, temperature, salinity, 

 tide, and currents. According to the quality of this evidence will rest 

 the conclusions as to the marine, estuarine, or lacustrine nature of the 

 deposition. 



It is evident that many criteria are subject to gradations in nature and 

 in clearness of development,^and in the following discussion of strati- 

 graphic characters the method will be to pass from those of less to those 

 of greater determinative value. 



ABSENCE OF FOSSILS 



This is a purely negative criterion, which by itself is of little value in 

 distinguishing between subaqueous and fluviatile terrestrial deposits. 

 The absence of molluscan life over considerable tracts of shallow sea bot- 

 tom has been pointed out recently by Kindle.^^ Even if life of some sort 

 has existed there are processes of abrasion and solution which may de- 

 stroy all evidences of it on the shallow sea bottom as well as on the land. 

 Oxidizing decay and solution are normally more effective in land de- 

 posits, owing to the circulation of air and ground water; but mechanical 

 destruction, through movements of surface materials, is more broadly 

 effective on a wave-worked bottom. The limestone muds of coral beaches 

 show how effectively a mass which originally was entirely composed of 

 organic remains may. by abrasion, solution, and redeposition come to be 

 almost barren of fossils. As a result of these agencies, unfavorable to 

 life or destructive to its remains, sandstones of either continental or 

 nuirinc origin are the formations most apt to be without fossils. 



Examples of such sand deposits accepted as continental may be cited 

 in the Triassic sandstones of both east and west and the sandstone mem- 

 bers in the Tertiary of the Eocky Mountain region. Casts of tree trunks 

 or silicified wood may reward careful search and calcareous phases tend to 

 protect animal skeletons from solution. But considering the abundance 

 of the past life which was associated with the deposition and the enor- 

 mous dominance of unfossiliferous beds, it is seen how accidental is the 

 preservation of fossils. Of marine sandstones, barren examples may be 



^3 E. M. Kindio : Cross-borldinfr and al)S(Mi('p of fossils considoivd as crUoria of conti- 

 nental deposits. American Journal of Science, vol. xxxii, 1011, pp. 22o-2o0. 



