CLIMATE AND TERRESTRIAL DEPOSITS 185 



change, this could not be carried down to all details; since it is seen, 

 for instance, that in Arizona, Queen Creek at the present time begins 

 to deposit sediment immediately upon leaving the mountains, while 

 the much larger Gila flowing parallel and but eleven miles to the south 

 flows through the plain as far as Florence in a well-marked valley. 

 In this respect it is to be expected that various streams will behave 

 somewhat after the manner of glaciers, where each responds in its 

 own time and to varying degrees to periods of increased or decreased 

 precipitation of snow. Third, while extremely contrasted climates 

 may leave niany distinctive marks in the character of the matrix of 

 even locally transported materials, as shown in the contrast of the 

 Carboniferous of Rhode Island to the Triassic of Connecticut, minor 

 climatic fluctuations cannot be expected to be so recorded. As evidence 

 of such variations, therefore, disturbance of the stream gradient and 

 changes in the coarseness of the detritus must be looked for. 



CONCLUSIONS ON RELATIONS OF CLIMATE AND EROSION 



The relative rates of erosion in desert, tropical, and polar climates 

 is a subject upon which there is much diversity of opinion, as Merrill 

 has shown.' The difficulties are largely due to the differences in 

 kind of erosion between hot and cold, and arid and rainy climates. 

 The usual geographic remoteness of these extreme types from each 

 other further increases the difliculty. Doubtful conclusions have 

 also sometimes been founded upon the quantities of rock waste 

 present, it being assumed that where rock is deeply decayed, as in 

 the rainy belts of the tropics, it now weathers and erodes rapidly, 

 whereas, on the contrary, the deep regolith may check further decay 

 and the matted vegetation retard the erosion of the surface. Or a 

 traveler may be impressed with the naked mountains and waste- 

 filled valleys of a desert region and conclude that here rock destruc- 

 tion progresses more rapidly than elsewhere on the earth. 



A more satisfactory method than that of founding conclusions on 

 quantitative estimates from a few unlike localities is to compare the 

 decay and erosion of unlike climates not with each other but with a 

 third term. For example, the subaerial erosion of arid and rainv 

 regions may be compared with the marine erosion of their coasts, a 



'■ Rocks, Rock-weathering, and Soils, 1897, PP- 278-85. 



