The Evolutiofi of Climates. — MiDison. 205 
THE EVOLUTION OF CLIMATES. 
By Maesden Manson, San Francisco, Cal. 
(Concluded from page 180.) 
(Plate XI.) 
Appencdix. 
The Columbiaii Lava Plain and its Climatic Influence. 
The Columbian lava plain occupies portions of the slopes of 
the Rocky, Sierra, Warner, Butte and Cascade ranges of 
mountains, and of the intervening valleys and plains. This 
lava-covered area is exceedingly broken and lies in the states 
of Washington, Oregon, Idaho, Wyoming, Nevada, and Cali- 
fornia, and is about 200,000 square miles in area. Its greatest 
length from the headwaters of the Big Horn river in Wyo- 
ming to the base of the Coast range in Oregon, is through 15" 
of longitude; and its greatest width is through 9° of latitude. 
It is interesting to note that the fractures from which this vast 
outburst occurred lie about the median line of the extreme lim- 
its of the North American land mass, and that about 180° from 
these fractures similar fractures occurred near the median line 
of the Euro-Asian land mass, giving rise to the great Deccan 
lava plains of India. The maximum thickness of the Colum- 
bian lava is known to be 4,000 feet, and many miles of this 
thickness can be observed. Vertical sections of marked dis- 
tinctness can be studied in the caiions of the Columbia river 
and its tributaries, and in the rifts known as Grand and Moses 
coulees. The aggregate length of deep river cafions and deep 
coulees which can be observed is several thousand miles, so 
that opportunities to study the formation and character of the 
lava are remarkably good. 
The sections are made up of as many as thirty successive 
layers of lava superimposed with varying intervals of time, as 
the joints are sometimes very distinct with no evidences of de- 
nudation nor sedimentation, and again contain hundreds of 
feet of fossiliferous strata. The layers are rarely over a few 
hundred feet thick and overspread subdivisions of the total 
area sometimes at close and sometimes at widely separated in- 
tervals. During these intervals the life of the period migrated 
back and forth over the cooling surfaces, and was de- 
