18, 3 Smith: Tropical Geology and Engineering 229 
second, because, when the velocity is doubled, it can carry par- 
ticles of rock sixty-four times as large as before, and in time 
of freshets bowlders weighing tons are carried along, each one 
acting as a battering ram. No retaining wall can be expected 
to withstand this terrific bombardment, nor does it do so. 
Again, the composition of the country rock, its structure, tex- 
ture, and the state of weathering are of vital importance; and, 
if this weathered material slides into the cafion below, a tempo- 
rary and very dangerous dam results. When the dam breaks, 
as it soon must with the impounding of the torrent behind 
it, all is swept before it. We have a remarkable example of 
this in the now famous Benguet Road on Luzon Island. Bued 
River is a small stream, little more than a creek in the dry 
season; it flows from 1,524 meters (5,000 feet) elevation to 
the sea. The valley walls are V-shaped, the average slope being 
probably as high as 35° or even 40°. The country rock is a 
badly decomposed andesite, for the most part with some tuff 
deposits, much jointed and with innumerable small faults, all 
loosened by many earthquakes. Here we have ideal conditions 
of instability, which were perfectly apparent to geologists; yet, 
so far as the writer knows, no geologist was consulted by the 
engineers until it was too late. The inevitable happened. First, 
slides continued until the slopes reached the angle of repose; 
then dams followed, impounding the water to a great depth; 
next, the dams broke and search parties were out looking for 
the road, buried under twenty meters of débris. Now, after 
many years and the expenditure of much money, a new road 
has been constructed by way of the old Naguilian trail. We 
have had to go back to the ridges, following the example of 
the Spaniards. Of course, if we had cared to, we might have 
taken a lesson from the native trails, the majority of which 
follow ridges. 
In the matter of engineering technic involved in the con- 
struction of these roads, the writer would venture to lay empha- 
sis on the necessity for drainage; keep the foundation of your 
road drained. The excellence and easy maintenance of the 
Hawaiian roads is due largely to the very porous subsoil and, 
consequently, perfect drainage. You may say that this is nothing 
new, that every engineer knows that; but the application of 
drainage in America, where the rainfall is merely a light sum- 
mer shower as compared with the tropical downpour, is a dif- 
ferent problem. As to how it should be drained, that is a matter 
not within the writer’s province as a geologist. 
