44 



map (Fig. 2) shows that the isotherms of 70 degrees for July and of 50 

 degrees for January cross near San Francisco and spread widely apart, 

 bounding a belt which belongs to the torrid zone in summer and the frigid 

 zone in winter. The climate is best characterized as intemperate, having 

 an annual range of 40 to 60 degrees. Maximum temperatures of 110 degrees 

 and mininiun) of -50 degrees are not unusual. The belt is swept by a 

 procession of cyclones and anticyclones which bring rajiid changes from 

 cool and dry to warm and wet and vice versa, two or three times a weelc. 

 The weather is iierhaps the most variable and uncompromising in the 

 world. Cold waves and hot waves intensify the seasons and give everybody 

 something to tallv and read about. The atmosphere furnishes a perpetual 

 turkish bath, running tlie ganuit from hot to cold and cold to hot in the 

 most stinuilating and irritating niaiuier. Our European friends say that 

 American hustle and restlessness and the strained expression on our 

 faces are due to the uncertainty and intensity of American weather. 



The eastern half of the intem})erate belt is saved from aridity by 

 cyclonic winds from the gulf and Atlantic, which carry a i-ainfall of 20 

 or more inches to Hudson Ray. The western half catches its moisture 

 as catch can and puts uft with the driblets left from the load dropped on 

 the eastern plains or the western mountains. The line near the 100th merid- 

 ian where the 20-inch isohyet and the 2,000 fw)t isohyps coincide is one of 

 the most strongly marked natural boundaries in the world. Tt is the west- 

 em limit of forest, prairie, agriculture without irrigation and dense popula- 

 tion. The medial belt of North America is divided into three pairs of 

 provinces, the Pacific, Interior, and Atlantic. The simplest is the In- 

 terior, including the Arizonan, already notictnJ. 



The Interior province is composed of two plateaus separated by the 

 broad system of the Rocky mountains. Tliere is not a square mile in it 

 below 2,000 except in th(> lower ('oluinl)ia valli'y. and most of it lies above 

 all but the summits of the Appalachians. Dn the west the smaller Co- 

 lumbia plateau is a frozen sea of lava, trenclied by the Snake and Colum- 

 bia rivers in ghnimy canons. Most of the scant rainfall sinks into the 

 crevices to reappear along the caiion walls in voluminous springs. The 

 dominant plant formation is sagebrush, which is neither grass nor shrub 

 nor tree, but .iust arteniesia. The eastern ])lateau, commonly known as the 

 Great I'lains, but better cIiaracterizcHl as the High Plains, is nearly 2,000 

 miles long and 300 to 500 miles wide. It is liroken here and there by 



