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



sand does not extend to any great distance from the present river chan- 

 nel, and hence the silt becomes less stable with increasing distance from 

 the river. 



The material which fills the tributary valleys, such as the Danta, the 

 San Francisco, the Cureho, and the Tamborcito, is a fine silt, generally 

 quite free from grit, with a blue color, and containing abundant frag- 

 ments of wood and leaves. When this material is thoroughly drained 

 it becomes fairly compact, as shown in the vertical banks of most of the 

 streams, but at some distance from these streams, where the drainage is 

 imperfect, it is quite soft to a great depth. 



This alluvial silt or mud when first exposed sometimes has a brilliant 

 blue color, which quickly changes to a yellowish brown on exposure to 

 the air. The change in the color takes place at the exposed surfaces 

 within a few minutes. 



The material forming the deltaplain of the San Juan is similar to that 

 composing its floodplains. The black sand is carried out to sea and 

 transported along the shore by littoral currents and thrown up to some 

 distance above tidelevel by the waves, so that within a belt two or 

 three miles broad along the coast the surface is composed chiefly of black 

 sand, with a small amount of vegetable mold. The fine silt increases 

 in thickness from a feather-edge at its outer margin at a rate somewhat 

 greater than the eastward slope of the deltaplain. 



It is probable that the delta has always been fringed by a belt of sand 

 which never rose more than a few feet above sealevel. The region, how- 

 ever, has been- sinking w^hile the delta was forming. As the delta grew 

 by accretions of sand to its outer margin, the corresponding growth on 

 its surface was made by the fine silt deposited from the flood waters of 

 the rivers. The plane separating the sand from the overlying silt thus 

 appears to have a gentle landward inclination, being slightly above sea- 

 level at the present coast and some distance below sealevel toward its 

 inner margin. 



RECENT VOLCANIC ROCKS 



The vulcanism which gave rise to the igneous rocks associated with 

 the Tertiary sediments appears to have become entirely extinct in this 

 region, and doubtless a long interval elapsed in which it was free from 

 any manifestations of volcanic activity. In comparatively recent times 

 the vulcanism was renewed and its products form the Costa Rican and 

 Mcaraguan volcanic ranges, which have already been described. Its 

 products also form the Jinotepe plateau and the plain of Leon, which 

 extend northwest from the Lakes to the Pacific. 



