sub-a£;rial alluviation. 257 



that an arid climate prevailed for a long time previous to the existence of 

 the Quaternary lakes, the records of which are now observable. The rate 

 at which alluvial cones are formed is irregular and depends on a number 

 of variable factors, as, for example, the amount of precipitation, the grade 

 of the canons, character of the rock forming tlie mountain, etc. As the 

 geological and topographical conditions at a definite locality may be con- 

 sidered constant, so far as the present discussion is concerned, it is evident 

 that alluvial cones are in some manner a record of rainfall. Many obser- 

 vations have shown that they are usually formed in arid regions, and result 

 from sudden storms which flush the canons and sweep out the accumulated 

 debris with violence. This is observable not only during the occasional 

 "cloud bursts," as the sudden storms of the Far West are called, but may 

 also be inferred from the occurrence of angular rocks, weighing many tons, 

 on the surfaces of the alluvial cones. During the intervals between the 

 storms, disintegration takes place in the uplands, and the smaller tributaries 

 deposit their loads in the larger canons, which thus become charged with 

 debris. The rapid deposition of alluvium about the mouths of canons is 

 largely influenced by the fact that what was entirely a surface stream 

 during its canon course, sinks below the surface on passing to the alluvial 

 slope and deposits its load. These considerations might be extended and 

 the action of perennial streams contrasted with the results produced by 

 infrequent storms, but perhaps enough has already been written to show 

 that alluvial cones are not only characteristic of arid climates but that the 

 precipitation which produced them is commonly paroxysmal. There is mani- 

 festly no uniform rate at which subaerial alluviation takes place and no 

 definite measure of the time necessary for the accumulation of debris of 

 this character, but the comparative size of the deposits made during distinct 

 periods furnishes at least a general indication of the relative length of time 

 required for their accumulation 



Assuming that the conditions of alluviation were equally favorable 

 during the pre-Lahontan and recent arid periods, we may determine from 

 the magnitude of the subaerial deposits in each instance the relative dura- 

 tion of the two periods. The Lahontan terraces carved on the slopes of 

 ancient alluvial cones are but delicate insci'iptions which, in a geological 



MON. XI 17 



