330 C. W. IIAYEH — GEOLOGY OF NICARAGUA CANAL KOUTE 



tained in the underlying rocks. This reducing action of the contained 

 vegetable matter prevents the oxidation of the iron in the alluvial silts 

 in the eastern division, and there seems no reason why it should not be 

 equally effective in preventing oxidation in the residual clays in the 

 western division. 



Another difference at once noted is the extent to which rock decay 

 has extended. The opportunities for determining the extent of rock 

 weathering on the west side have not been so good as for determining 

 its extent in the eastern division, and the rocks which are there present 

 do not afford the same variety in composition and structure. Observa- 

 tions are confined practically to two kinds of rock, namely, ^le igneous 

 basic rock forming the large area north of the Rio Grand valley, and the 

 rocks of the Brito formation. The basic igneous rocks do not differ 

 essentially from those which occur on the east side, Avhere they are 

 covered with a great depth of red and blue clays. On the west side, 

 however, the residual material covering them consists of a comparatively 

 thin layer of bluish gray clay. It is somewhat doubtful whether the 

 thinness of this residual mantle is due to the less rapid decay of the 

 rock or to the more rapid removal of the products of weathering. Cer- 

 tainly the latter factor is important, but the rate of weathering may also 

 be very much slower under the climatic conditions which here prevail 

 than in the eastern division. The blue clay appears to constitute j^rac- 

 tically the only product of decay, and the extensive zone of soft rock in 

 which the minerals are entirely altered, but in which the original rock 

 structure remains, is wanting. 



The clay derived from the deea}' of the Brito formation is quite sim- 

 ilar to that derived from the igneous rocks, except that it contains a 

 notable amount of sand where it is derived from the more sandy por- 

 tions of the formation. Where derived from the calcareous shales, it 

 forms a blue or black tenacious, plastic clay. Its depth varies from 

 nothing up to 10 or 15 feet, depending on the position in which it 

 occurs. The greatest thickness is found in the level valleys, where the 

 surface is practically at baselevel, and where the surface erosion is very 

 nearly reduced to zero. On the steep hillsides, on the other hand, the 

 same kinds of rocks are covered with a very scant}' layer of residual soil 

 or it may be entirely wanting. 



So far as known, there is nothing on the west side which corresponds 

 to the zone of soft rock generally represented in the sections from the 

 eastern division. Wherever opportunitj' was afforded for observing the 

 character of the passage from the overlying blue clay to the underlying 

 igneous rocks, the transition was found to be abrupt, and the interme- 

 diate zone of weathered rock was absent. 



