306 GEOLOGY OF THE BLACK HILLS. 



this eastern slope of the Hills has a greater rain-fall than the western, judg- 

 ing from the growth of plants and trees, a fact which would coincide with 

 the theory that the rain-producing winds came from the east and precipi- 

 tated their moisture on that slope. 



But the showers of rain and the thunder-storms which we so frequently 

 experienced seemed to be independent of the easterly winds as a source of 

 their moisture, and to be local in their origin and occurrence. Generally 

 of short duration, they came usually from the west and passed rapidly by, 

 raining hard for ten to twenty minutes, and leaving the grass and trees 

 dripping with moisture and refreshed by their passage. 



The winds which blow on the Hills from any direction pass over the 

 plains for a great distance, and, gathering some moisture, part with it on 

 being chilled by contact with the cold and elevated peaks in the interior. 

 Thus the Black Hills may be said to have a local and limited rain-fall not 

 directly dependent on the easterly winds, which, nevertheless, furnish the 

 greater supply of moisture. 



The rain-fall experienced in the Hills during the progress of the survey 

 was very remarkable for the frequency and regularity of the showers and 

 their short duration. It was varied by the occurrence of occasional long, 

 steady rains, and damp, drizzly, or foggy days. From June 1 to August 1 

 it rained more or less at least twenty days in the month. Usually the sky 

 would be overcast by clouds about 3 or 4 o'clock in the afternoon. A smart 

 shower, lasting from a quarter to half an hour, would rapidly pass by from 

 the west, and the sun would shine again and quickly dry up the ground. 

 On one occasion I experienced, near Harney Peak, four distinct showers, 

 accompanied by thunder and lightning, in a single day, which was otherwise 

 warm and pleasant, the sun shining brightly except when it was raining. 

 It was not unusual to have a shower during the day followed by another 

 after midnight. After August 1 the showers were not so numerous and the 

 rain-fall was appreciably less, although several rainy days occurred in both 

 August and September. 



Referring to my note-book, I find the following record of the rain-fall 

 observed while prospecting on Rapid and Spring Creeks : 



June 20, rained steadily all the afternoon. 



