1 64 STUDIES FOR STUDENTS 



will notably modify the character of the sediments laid down upon 

 the latter. It is necessary, therefore, to consider geographic factors 

 as well as climatic, and to begin at the river's source. The geographic 

 and topographic relations in the regions of erosion existing during a 

 distant epoch cannot, however, be directly ascertained by observation 

 of the existing rocks, as may the conditions in the region of deposition. 

 These relations may, furthermore, in their effect upon the alluvium, 

 simulate and mask the climatic effects to some extent. As a result 

 it is impracticable to make fine distinctions in climatic cause over 

 the regions of erosion, and in general only those types of climate 

 which markedly affect the quantity and kind of land waste need be 

 considered. These may be classified as of four types, as follows: 

 warm and rainy, warm and arid, cold and rainy, cold and arid; truly 

 glacial climates being excluded from the scope of the paper. 



Character of Rocks Supplying Sediment 

 similar effects from lithologic and climatic causes 

 In alluvial deposits the nature of the rocks supplying sediment is 

 a matter of importance. A basic igneous rock, for instance, even in 

 a subarid climate, such as exists over the Deccan of India and the 

 northwestern United States, may give rise to a large amount of 

 ferruginous clay containing mineral fragments other than quartz, 

 and of various sizes. After sufficient transportation and sorting 

 such clays would be the dominating deposits, and except for a greater 

 content of lime, must closely simulate clays from well-decayed nor- 

 mally acidic rocks of a more pluvial climate. Again, a region of 

 calcareous rocks will furnish a large amount of lime in solution, and 

 this, by cementing the alluvium of the delta, as for example, in that 

 of the Rhine, may cause the river deposits of a humid climate to 

 resemble in this respect the alluvium of subarid regions. Further, 

 the material derived from rocks of a sedimentary nature owes part 

 of its character to a previous cycle of erosion and deposition under 

 different climatic and geographic conditions. 



ELIMINATION OF LITHOLOGIC FACTORS 



This possibility of confusion between climatic and lithologic influences 

 over the region of the headwaters may be obviated to a large extent, 

 firrst, through the determination of the source of sediments and, if 



