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



Blite day. — This division is usually somewhat thicker than the over- 

 lying red clay. While its prevailing color is blue, it varies from white to 

 various shades of yellow and brown, depending largely upon the original 

 composition of the rock from which it is derived. It represents the zone 

 of complete rock decay and disintegration but incomplete oxidation. 

 The blue color is due not to the presence of a reducing agent, but to the 

 absence of a sufficient oxidizing agent to convert the iron into the higher 

 oxides. It generally contains more or less abundant fragments of thor- 

 oughly weathered rock, which retain their original structure, and where 

 it is derived from basalt it usually contains numerous boulders of fresh 

 rock, the nucleii about which concentric weathering has taken place. 

 The lower limit of the blue clay division is often more indefinite than 

 its upper limit. By an increase in the number and size of the rock frag- 

 ments, both fresh and weathered, it passes into the zone of soft rock. As 

 will be readily seen, the point at which the division should be drawn is, 

 to a large extent, arbitrary, since the distinction is at best only one of 

 degree. 



Soft rock (saprolite). — The red clay retains but few of the character- 

 istics of the rock from which it was derived ; hence it is fairly uniform 

 throughout the region. In the blue clay, also, the original character of 

 the rock is almost entirely obliterated, and it is therefore somewhat uni- 

 form. In case of the soft rock, however, in so far as it retains the original 

 structure of the rock from which it was derived, it presents the same 

 diversity as the hard rocks of the region. In some cases this division is 

 wanting, and the blue clay extends entirely down to the fresh rock. This 

 is the case with the Machuca sandstone. In other cases the blue clay is 

 thin or absent, and there is a great thickness of soft rock. This is usually 

 the case with the dacite. 



The material classed as soft rock represents the zone of practically 

 complete rock weathering, but of incomplete rock disintegration. The 

 forms of the constituent minerals can usually be made out in rocks 

 which were originally coarse grained. The original structure is gener- 

 ally well preserved. In the vesicular lavas the gas cavities are nearly 

 as perfect as in the hard rock. In the volcanic conglomerates and 

 breccias the distinction of matrix and inclosed pebbles or angular frag- 

 ments is perfectly sharp, yet all the material included in this class can 

 be crumbled in the fingers. 



The extensive beds of fine basaltic and andesitic tuff which occur in 

 the Eastern divide and elsewhere are perhaps the most easily altered 

 rocks in the region. There is some doubt as to their ever having been 

 thoroughly consolidated, and this may account for the depth to which 

 they are weathered. Wherever found, even under a great mass of com- 



