1906] SHANTZ—VEGETATION OF THE MESA 21 
PHYSIOGRAPHY AND SOILS. 
The exposure of the Mesa is southeast, the grade being about 
18™ to the km. The northwest portion, which is the highest, has an 
elevation of 1889™. On the east and south sides of this Mesa the 
water has cut deep gullies (fig. 1), which appear older on the south than 
on the east, the slope being more gradual and the soil more stable. 
The east side of the Mesa is bounded by a low region largely of 
clay sand, which slopes gradually to Monument Creek. On the 
north side there is a less elevated region which, however, does not 
differ markedly from the Mesa itself. On the west side is Camp 
Creek, which has cut down into the Fort Pierre clay. 
The soil of the Mesa is a gravel mixed with a. limited amount 
of clay and humus. The gullies and edges of the Mesa are made 
up of Ft. Pierre clay, which is in places mixed to some extent a 
the Quaternary gravel which lies above it. 
CLIMATE. 
Rainjall_—The greatest amount of rain is during the growing 
season, the fall and winter, as a rule, receiving very little. As a 
result the vegetation is not protected in the least by snow during 
winter, nor is there a sufficient amount of water to retard the evapo- 
ration from the aerial parts of the plant. There is, as a rule, con- 
siderable rain during the summer months from May to September, 
but often the rainy season is much shorter, covering, as it did in 
1903, only June, July, and August. The rainfall is about 32 to 
43°™, but because of the unequal distribution throughout the year, 
this affords a rather luxuriant summer growth. This seasonal 
variation in rainfall is best illustrated by the following table, which 
gives the rainfall in centimeters. 
Year | Jan. | Feb. |March| April |. May | June | July | Aug. | Sept.| Oct. Nov. Dec. | Total 
TOGT | O.5§ | 6.57 12.07 | 4.67 | 0.52 4 sts. | on one e508 PS-58, | Bees 0.02 | 0.45 tees 
1902 | 0.20 | 0.35 | 1.01 | 2.07 | 13.28 | 3.06 | 4.21 | 6.83 | v.40 | 0.38 | 9.05 | 0.55 | 33.20 
1903 | 0.15 | 1.77 | 0.93 | 2.41 | 1.57 |12.95 | 1-07 | 6.29 | 1-52 | 0.86 | 2.03 | 0.63 | 32.18 
1904 | 0.27 | 0.48 | 0.15 | 0.58 | 10.46 | 9.86 | 7.87 | 6.35 | 5.25 | 9-67 | Trace 
There is also a daily variation in rainfall which is of some impor- 
tance to the plant. The relative humidity, of course, is much higher 
‘during the night than it is during the day, and on this account rain 
