The Shinumo Area. 379 



phenomenon that contributes to the greater rainfall of the 

 north rim seems to be the presence of the Grand Canyon itself : 

 after every general storm that visits both sides of the canyon 

 alike, there follows on the south rim a day of clearing ; but the 

 clouds that rise out of the canyon after the storm sweep back 

 over the north wall and re-precipitate on the Kaibab surface ; 

 these secondary storms almost never return over the south rim. 

 The climate of the Coconino Plateau in the area is semi-arid ; 

 often no rain will fall for a month at a time. The precipi- 

 tation is greatest in the winter months and in the month of 

 July. Within the canyon much of the rainfall evaporates 

 before it reaches the greater depths, so that the climate of the 

 lower part of the canyon is more arid than that of the south- 

 ern plateau. 



Powell Plateau, whose higher eastern portion has an altitude 

 equal to that of the Kaibab rim, and whese lower western por- 

 tion has an altitude equal to that of the Coconino, has an inter- 

 mediate climate. Its situation as an island in the canyon series 

 to moderate the cold in winter, for the warm air, rising out of 

 the deep canyons that surround it, acts as a radiator : the snow 

 does not accumulate so deeply on the eastern end as on the 

 Kaibab ; and on the western end does not accumulate at all. 

 Its surface is a resort in winter for the game and wild horses 

 that are driven out of the Kaibab by the snow. Its exposed 

 position subjects it to violent gales of wind at all times of the 

 year. The higher eastern end receives an abundant rainfall, 

 while the lower western end is semi-arid. 



The variation of the flora in the Shinumo area is as great as 

 that of the climate. The surface of the Kaibab Plateau is 

 covered with a magnificent open forest of yellow pine ; the 

 trees grow large and far apart and the ground is free from 

 undergrowth, giving its surface the aspect of a great park ; 

 Englemann spruces grow on the north slopes of the washes, 

 and cottonwoods, aspens, and scrub oaks in their bottoms ; a 

 minor flora of flowering plants, exceedingly rich in species, 

 covers the floor of the forest. The flora of the plateau surface 

 of the south rim of the canyon differs completely from that 

 of the Kaibab ; it is covered with a forest of gnarled and 

 stunted trees of juniper and pinon, with here and there a buck- 

 brush bush ; the trees .never form thickets, but grow wide 

 apart ; while the open stretches are covered with sage brush 

 and mormon tea, with occasional cactus, mescal, and plants of 

 the century family. This difference between the floras of the 

 north and south rim is due to the differences in precipitation 

 and temperature, which vary directly with the altitude. For 

 this reason the floras of the plateaus furnish an almost unfail- 

 ing index of the elevation. This is beautifully shown on the 



