190 STUDIES FOR STUDENTS 



find a delicate response in the rates of erosion and the kinds of material 

 supplied to the streams. With each climatic oscillation the delicate 

 balance of erosion to stream transportation is thrown out of equilib- 

 rium and a wave of disturbance originating at the headwaters is sent 

 to, and even beyond, the river's mouth. Even when the nature of 

 the climatic changes at the distant source cannot be determined, 

 they should still be recognized as possible factors in those lithologic 

 distinctions which characterize the successive stages of a sedimentary 

 formation. 



The influence of climatic changes upon sedimentation has been 

 tacitly recognized by certain leading writers, either as a hypothesis 

 to account for a regular alternation of marine strata, as has been done 

 by Gilbert;^ or, through the use of gravel terraces as a means of 

 correlation between glaciated and unglaciated regions. In the latter 

 case they have been distinctly recognized and classified by Davis 

 and Huntington as terraces of climatic origin, and criteria for their 

 distinction have been devised. The relations of climate to erosion 

 appear to be so sensitive, however, and so important, as a causal 

 factor in the variations of stratified rocks, that it would seem desirable 

 to distinguish it clearly as a separate cause, and call special attention 

 to it, as is here done, as of co-ordinate importance with minor move- 

 ments of the earth's crust. The lack of conscious recognition of this 

 factor has without doubt caused many minor sedimentary changes, and 

 even some greater ones, to be taken as evidence of earth movement, 

 when climatic changes have at least entered as important contributory 

 factors. 



Before these waves of climatic effect find record in the strata, 

 however, they are modified by the accompanying variations in the 

 power of transportation and the changes which the sediments undergo 

 before burial on the surface of the flood-plain or bottom of the shallow 

 sea, modifications which will be discussed in the two succeeding parts. 



I "Sedimentary Measurement of Cretaceous Time," Journal of Geology, Vol. Ill, 

 1895, pp. 121-27. 



\To he continued] 



