20 



H. E. Gregory — A Geologic Reconnaissance 



ing records of precipitation indicate that the number of rainy 

 days in a year averages about 175. May, June, July, and 



Precipitation records, Cuzco, collected from various sources. The figures for 

 the year 1895 are considered typical. 



Year 



Jan. 



Feb. Mar. ; Apr. 



May June 



July 



Aug. Sept. 



Oct. Nov. 



Dee. 



Annual 



1894 











0.01 



0.99 



3.58 



3.18 



3.21 





1895 



7.01 



8.25 I 5.18 2.68 



0.90 , 0.91 



0.29 



1.36 



1.40 



4.53 



2.19 



6.57 



41.07 



1909 



8.34 



2.13 j 1.93 9.09 



1.96 



0.28 



0.41 





1.15 



3 09 



1.98 



:;<). 30 



1910 | 2.49 



2.23 : 5.86 ; 3.89 

















August are practically without rain. Weekly rains with 

 cloudy skies are usual for September and October, also for 

 March, April, and to a less extent for May — the autumn 

 months of the Southern Hemisphere. The rainy season begins 

 in November with daily thunder showers, and during January 

 and February precipitation is, in normal years, almost con- 

 tinuous. The discharge of streams during the Peruvian sum- 

 mer is therefore continuous and of large volume, and inasmuch 

 as the cover of vegetation is scant and patchy, particularly on 

 slopes of intermediate height, the run-off is only slightly 

 retarded and vigorous erosion is favored. The sudden fluctua- 

 tions of streams, due to the heavy, short-lived downpours of 

 spring and autumn, also facilitate the removal of rock debris. 

 As the result of a shower on October 23, 1912, the flow in 

 the Huatanay increased from about 0.30 second-foot of clear 

 water to approximately 8 second-feet of water containing 12 

 per cent of silt. On the morning of October 29 the Chun- 

 chullumayo was a trickling rill, clear except for ever-present 

 sewage; a violent thunder shower accompanied by hail con- 

 verted the stream into a torrent which undermined banks and 

 redistributed the gravel bars. After the lapse of two hours 

 the water, carrying 18 per cent of suspended red mud, had 

 decreased one-half in amount; next morning the stream had 

 assumed its normal appearance. The relation of run-off to 

 effective erosion is thus seen to be not unlike that prevailing 

 in arid regions. 



The temperature of the Cuzco Valley, and hence the effective- 

 ness of rock disintegration from frost, is subject to wide varia- 

 tions. The mean monthly temperatures range from about 41° 

 in July to 54° in November, and the contrast between day 

 and night temperatures is particularly marked. Extreme cold 

 is unknown, but snow on the mountains and ice on standing 



