Some Engineering Problems Of The Panama Canal In Their Relation To Geology And Topography

12 ENGINEERING PROBLEMS OF THE PANAMA CANAL. The hill land of the Canal Zone will never have much agricultural value, even for tropical products, because of the steepness of the slopes and the shallowness of the soil. The steep land if cultivated would wash rapidly in the heavy tropical rains of that region. A subsequent discussion of the effect of streams brings out the fact that erosion took place in the upper parts of stream valleys, whereas deposition of gravel, sand, and silt was the dominant process in the lower ends. Now, in nearly all countries the broad flood plains that have been built by deposition of ricli silty soil are especially adapted for agriculture. The flood plains of tho Chagres, Trinidad, and Gatun Rivers contained much rich land suitable for growing bananas, tobacco, and sugar cane. However, they also contained pools of stagnant water that afforded breeding places for mosquitoes and other posts, and hence were considerably more unlioalthful than the higher ground. Much of this alluvial land lias now been, covered by Gatun Lake, and what remains will probably never have value commen- surate with its richness of soil because of the depressing climate and the malarial conditions that prevail. Most of tho timber within the Canal Zone was cut years ago, so that nearly all the lumber needed for building the 11 construction- camp” towns had to bo imported. Some of the ties for the Panama Railroad were made by natives from extremely resistant native woods, such as guyacan (or false lignum-vitæ) and nîspero, or balata tree (Mimusops darinensis Pitticr). The name, however, is also applied to other trees of the same family. For making dugout canoes the natives use chiefly espavé (Anacardium rhinocarpus D. C.), cedro, or Spanish cedar (Cedrela sp. pl.), and pochotc (Bombax Barrigon Seem.). In general, tho forests were more of a hindrance to the work than otherwise, for they formed a thick jungle through which many survey lines had to bo cut and clearings made for roads, trails, reservoirs, and town sites. STREAMS. The heavy precipitation is evidence that the country must be well watered. The Chagres, which flows for a part of its course through the Canal Zone, is the chief stream (see Pl. IV). It has a relatively large catchment basin, the slopes of which are locally steep; hence during heavy rains the volume of its flow is vastly increased, only to shrink greatly during the dry season. Trickling streams swell into destructive torrents after long and heavy rains. Tho percentage of absorption during the wet season is relatively small because the covering of porous soil, especially on the higher land and steep slopes, is very thin, and most of the rocks, except the river alluvium and gravels, the conglomerates, and sandstones, are exceedingly fine grained and have relatively little pore space or