18, 3 Smith: Tropical Geology and Engineering 225 
increasing its transportation power sixty-four times, we realize 
what a power we are trying to combat. We shall return to this 
topic. 
Once the tropical downpour of warm rain has stripped off 
the soil, weathering can and does strike deeper into the core of 
rocks beneath, and however far we go underground in mine 
workings we find the rocks exhibiting incipient decomposition. 
The writer knows this to be true from the examination of a great 
many thin sections of wall-rock from the deepest mines in the 
Archipelago. Undoubtedly the presence of organic acids, result- 
ing from the decay of the rank tropical vegetation, hastens 
this decomposition. Of course, we have little or no frost, except 
in the highest mountains, but we do have wide ranges in temper- 
ature during the twenty-four hours and this adds to the disinte- 
grating forces at work. Therefore, all things considered, we 
have reason to think that gradation, which includes degrada- 
tion, is the most potent of the geologic processes at work in the 
Tropics. 
This exessive weathering in tropical regions, where andesite 
and basaltic lavas are to be found, has resulted in places in the 
accumulation of a formation, of considerable thickness, known 
as laterite. This is an aluminous soil, or heavy clay, also rich in 
iron. When the iron exceeds 35 per cent it can be used as an ore. 
Vast deposits of laterite are known in India and Cuba, and some 
years ago an American engineer discovered a commercial deposit 
of this in northeastern Mindanao, in the Philippines. A study 
of this deposit shows that it is a product of weathering and con- 
centration. 
The work of organisms.—Nothing has been said as yet about 
the destructive work of plants and animals in the tropical re- 
gions, as the writer has made little personal study of the subject. 
There is no question but that they play no inconsiderable part 
in the processes of weathering and on occasion have to be reck- 
oned with in engineering operations. We have but to call at- 
tention to Branner’s ? observations on the geologic work of ants 
in Brazil to remind the reader of the importance of these insects. 
Not only do their numerous mounds, in some cases exceeding 
5 meters in height, considerably alter the topography, but they 
seriously undermine the subsoil so as to endanger structures. 
They promote weathering by opening the formations to the 
atmosphere and to gases. The writer has seen many ant hills in 
*Branner, J. C., Bull. Geol. Soc. Am. 21 (1910) 449-496. 
