250 The National Geographic Magazine 



5. In many places no deposits from 

 rivers are seen. If there are no deposits, 

 there can have been no erosion. The 

 amount of deposit discovered must equal 

 the amount eroded, for none is carried to 

 the sea or otherwise hidden. 



6. The water in the middle of a'stream 

 moves faster than that near the bottom 

 or sides ; hence the upper layers move 

 over the lower layers, and the latter thus 

 become nearly stagnant, and lose what- 

 ever cutting power they might have pos- 

 sessed. 



7. Water, being liquid, flows around 

 and over obstacles instead of cutting 

 them away. The existence of an island 

 in a stream is conclusive proof of the 

 stream' s inability to erode. Being liquid, 

 water cannot hold up its cutting tools 

 to their work. 



8. The fluency of water diminishes 

 with the amount of sediment carried. 

 Since a river has no means of getting 

 rid of its load of detritus, this load ac- 

 cumulates near the mouth, where it must 

 eventually become too great to allow 

 erosion ; hence stream erosion, if there 

 be any, must be confined to the upper 

 reaches of the streams, where the load 

 of detritus is moderate. 



9. True, flowing water does change 

 the form of canyons carved by other 

 agencies. Thus it changes the cross- 

 section of a glacier-carved valley from a 

 U shape to a V shape, but we will not 

 call this erosion. 



10. It is perfectly possible for streams 

 to carry sharp- edged sand along their 

 bottoms and sides without doing any 

 erosion. 



11. If one cannot prove absolutely 

 that erosion is in any case caused by 

 running water, therefore it must have 

 been done by ice. 



12. The majority of geologists and 

 physiographers are in accord with these 

 views. H. G. 



Field Courses in Geology ♦ — A joint an- 

 nouncement has been issued describing 

 the field courses in geology which will 



be given during the summer of 1905 by 

 Chicago, Columbia, Harvard, Johns 

 Hopkins, Kansas, Minnesota, North 

 Carolina, Ohio State, Iceland Stanford 

 Junior, and Wisconsin Universities. 

 There is also an inter-collegiate Appa- 

 lachian course of five weeks' duration, 

 which will be given under the direction 

 of several instructors, and will include 

 the study of the tertiary and cretaceous 

 formations of Maryland, the paleozoic 

 strata of the Susquehanna-Juniata dis- 

 trict of Pennsylvania and central New 

 York, the crystalline and paleozoic rocks 

 of the Little Falls district of eastern 

 New York, and the metamorphic and 

 triassic rocks of western Connecticut. 

 The courses offered by the several uni- 

 versities cover a wide range of territory, 

 extending from Vancouver Island and 

 California, on the Pacific coast, to 

 North Carolina, Maryland, and New 

 York, on the Atlantic, while one course 

 is announced for Iceland, where four 

 weeks will be spent in the study of 

 volcanoes, glaciers, and geysers. Each 

 of these courses will be under the guid- 

 ance of a geologist familiar with the 

 geology of the region studied. In the 

 list of instructors appear the names of 

 the following well known geologists : 



J. C. Branner, Wm. B. Clark, H. P. 

 Cushing, Wm. M. Davis, A. W. Gra- 

 bau, C. W. Hall, E. Haworth, W. H. 

 Hobbs, Charles S. Prosser, R. D. Salis- 

 bury, N. S. Shaler, Stuart Weller, J. B. 

 Woodworth, T. C. Hopkins, S. Barrell, 

 R. T. Chamberlin,W.W. Atwood, T. A. 

 Jaggar, Collier Cobb, and J. F. New- 

 som. 



THE ECONOMIC IMPORTANCE OF THE 

 PLATEAUX IN TROPIC AMERICA 



BY J. RUSSELL SMITH, PH. D. 



Tropic America presents the unusual specta- 

 cle of a region in which one type of district 

 supports most of the population and another 

 supports the more important foreign trade. 



In temperate North America and in Europe 

 the centers of population and production are 

 upon the lowlands. In tropic America the 

 centers of population are upon the highlands, 



