SEDIMENTATION, ENVIRONMENT, AND EVOLUTION. 17 



stages of the interpretation of past conditions seems to be 

 proved by the recent work of the American geologists, both 

 on the ancient strata and on the special characteristics of forma- 

 tions being built up at the present time. When the study of 

 modern sediments, aqueous and continental, has become 

 more intensive, we shall doubtless be able to discover criteria 

 at present unrecognized in the rocks of various ages, and to form 

 in consequence more exact ideas as to the mode of origin, 

 climate, and rainfall, relation to shore-lines and so forth.* 



Indications of the environment at the time of formation 

 of rocks may be yielded along the following lines : — 



(1) The Medium. Air or water, as indicated by the 

 fossils themselves. 



(2) Depth of Water, as shown by the lithology of the 

 deposit and the character of its fauna. Considerations of light 

 and motion also arise here. 



(3) Climate (by comparison with the present day), as 

 indicated by (a) organisms, (b) lithology and petrology of the 

 rock. Deposits laid down under glacial, temperate, sub-tropical 

 and tropical conditions, in deserts or derived from deserts, and 

 under the effects of heavy rainfall, each bear well-marked 

 characteristics by which they can be recognized. These 

 characteristics may be physical, as in the formation of drei- 

 kanter and rounded grains under desert conditions, and of 

 striated and far-travelled rocks as a result of glacial action ; 

 or they may be chemical, as in the differing degree and nature 

 of alteration of the constituent minerals of deposits formed 

 by glacial action, under arid conditions or those of moist heat.f 



* Since the above was written, many papers read at a symposium on 

 sedimentation by T. W. Vaughan, C. Schuchert, H. E. Merwin, E. W. Shaw, 

 and others, have been published in the Bull. Amer. Geol. Sec, Vol. 31 (1920), 

 p. 401. 



f See, for example, Barrell, J., " Relations between Climate and Ter- 

 restrial Deposits," Joum. of Geology, Vol. 16 (1908), p. 159, and "The 

 Climatic Factor as illustrated in Arid America," by Ellsworth Huntington, 

 with a contribution by C. Schuchert on " Climates of Geological Time," 

 Publication 192, Carnegie Institution, Washington, 1914. 



