322 C. W. HAYES — GEOLOGY OF NICARAGUA CANAL ROUTE 



probably not very great. Near the centers of eruption the character of 

 the tuff is somewhat different from that at greater distances. It is less 

 homogeneous in character and frequently consists of sharply defined 

 alternating tuff beds which differ widely in appearance. In the bluffs 

 surrounding lake Apoya this is well shown. Numerous distinct bands 

 of white pumice occur interbedded with dark lapili and fragmental 

 rocks, and these in turn are interbedded with the lava flows. 



Rock Decay 



importance of the subject 



One of the features which first impresses the geologist or the engineer 

 in Nicaragua is the extent to which the surface rocks are weathered. 

 This feature is common to all tropical regions, at least to those in which 

 there is an abundant rainfall. While the extent of rock weathering has 

 an important bearing on the geology of the country, and thus a high 

 degree of scientific interest, it is a fact of prime importance to the engi- 

 neer in planning any structures in this region. It enters directly into 

 the cost of excavation and also into the cost and permanence of foun- 

 dations for all heavy structures. 



CONDITIONS FA VORING ROCK DECA Y 



General discussion of the subject. — It has been shown by various investi- 

 gators that the conditions most favorable to rapid rock decay, and hence 

 to the accumulation of an extensive mantle of residual materials, are 

 high temperature and abundant moisture. These conditions are only 

 indirectly responsible for a large part of the rapid rock decay which 

 always accompanies them. They are also the conditions on which the 

 rapid growth and decay of a luxuriant vegetation depends, and it is the 

 latter process which is chiefly instrumental in hastening the process of 

 rock weathering. 



It is manifest that heat alone, without moisture, does not give rise to 

 conditions which favor rock decay,for it is a common observation in desert 

 regions, where the temperatures reach the maximum, that the rocks are 

 disintegrated to a limited depth by the alternate expansion and contrac- 

 tion due to changes in temperature, but that rock deca}'' is practically 

 absent. On the other hand, abundant moisture and continuous low 

 temperature do not give conditions favorable for rock decay, since these 

 conditions favor the accumulation of ice and snow. Glaciers are effect- 

 ive instruments for transportation of rock debris and to a limited extent 

 are efficient as eroding agents, but practically no rock weathering goes 

 on in their presence. 



