500 SEE— ORIGIN OF HIMALAYA MOUNTAINS. [April i8, 



runs westward through India, and down the east shore of Africa 

 to the Cape of Good Hope, it is observed in each of the four con- 

 tinents traversed that where the plateau is highest it usually narrows 

 in width, and vice versa. 



Thus we have seen that the plateau of the Andes is high in 

 Chile, Bolivia, Peru and Ecuador, but in Colombia falls to about 

 half its former level, and expands to about double width. This 

 expansion of the width of the plateau in Colombia is characteristic 

 of plateau formation in general. There are slight exceptions to the 

 rule, but the conformity to it is much more noticeable. For 

 example, at Titicaca the width is about 250 miles, but some distance 

 north of this region the Andean Plateau seems to narrow up till the 

 width scarcely exceeds 150 miles, in Ecuador; but it then spreads 

 out again as the range enters Colombia. 



It is not easy to explain this narrowing of the range, unless the 

 great width and great height at Titicaca are due to the indentation 

 of the coast at this point, giving uplifting forces from both direc- 

 tions, at the same time. This explanation seems to be well founded, 

 and is confirmed by the corresponding efifect north of central India, 

 where the plateau of Tibet reaches its maximum elevation. 



Accordingly, we probably should conclude that the width of the 

 Andean plateau is normally less than at Lake Titicaca, and that the 

 width there is due to a combination of forces from the two lines of 

 coast, meeting at an angle of about 135°. It is therefore a fact in 

 South America that wherever the plateau is widest, it decreases in 

 elevation, as in Colombia. 



In this problem of uplift, however, something depends on the 

 depth and width of the adjacent elevating ocean, and thus a certain 

 amount of variety should result. Since the adjacent sea is not of 

 uniform effectiveness, we should expect minor deviations from the 

 law ; but obviously they should not be too pronounced. 



In North America, the same general law holds true. Wherever 

 the plateau is narrow, as in central Mexico, the elevation is great ; 

 but where it is wide, the elevation generally is lower. There are of 

 course some exceptions to the rule, but it generally holds true. 



For example, along the Rocky Mountain range the highest part 



