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



cult to detect. From the character of the exposures in the bluffs along 

 the San Juan river, and from the results of the drill sections, it seems 

 probable, however, that the bulk of these fragmental rocks is as great or 

 greater than that of the massive rocks. About 4 miles above the Boca 

 San Carlos these beds are exposed in a higli bluff on the north bank of 

 the river. There is shown a considerable diversity in the character of 

 the material, varying from the finest tuff to coarse rounded conglomer- 

 ate. All parts of the beds are equally weathered, forming a tough clay 

 quite free from grit. The different beds vary considerably in color, 

 although the prevailing colors consist of various shades of red and 

 brown. The planes of stratification between the different beds are not 

 sharply marked, and the indications are that the deposit was made 

 rather rapidly and in the presence of strong currents. Similar exposures 

 of thoroughly decayed sedimentary beds occur in the river bluffs at 

 various points between Ochoa and the mouth of the San Francisco. 



It is probable that during the extrusion of the volcanic rocks in this 

 region numerous bodies of water were formed by the interruption of 

 drainage lines by the lava flows. In these bodies of standing water the 

 finer tuffs were accumulated with considerable regularity in their planes 

 of stratification. Forests were present on the adjoining shores and much 

 vegetable matter was accumulated along with these silts. There thus 

 resulted deposits of considerable depth, such as those encountered at 

 Lower Ochoa. These were subsequently covered by lavas or deposits of 

 fragmental material, but have never been buried sufficiently deep to 

 bring about their complete consolidation. 



In some places conditions were favorable for the deposition of calca- 

 reous material. In the section at the San Francisco a bed of very fine 

 grained earthy limestone about 3 feet in thickness was encountered with 

 fine tuff above and below. The limestone was perhaps originall}^ a cal- 

 careous mud which has been thoroughly solidified and is now compara- 

 tivel}^ hard. The adjacent tuffs, both above and below, may have been 

 solidified at one time, but are now soft and talcose. No traces of organ- 

 isms can be detected in this limestone, and it may have been precipitated 

 from solution without the intervention of life. In the railroad cut near 

 Silico lake there occurs a bed of clay enclosing water-worn pebbles and 

 numerous fragments of wood which is immediately overlain b}^ a flow 

 of basaltic lava. This cla}'' was doubtless alluvial or accumulated in a 

 lake and has probably not been buried to a sufficient depth to produce 

 consolidation. 



The beds of lava and volcanic tuff above described have been but little 

 changed from the position in which the}'' were originally deposited. 

 Wherever bedding planes can be detected in the stratified tuffs they are 



