A NICARAGUAN SHELL-BANK. 95 



to the shores, and the fossil shells no doubt were developed 

 under similar conditions. 



The conclusion naturally follows that at one time the San 

 Juan River and, of course, Lake Nicaragua, of which it is the 

 outlet, had their low water mark more than fifteen feet above 

 the present level; that the river has since cut through the 

 eastern barriers to its present level, in its downward progress 

 gradually depositing an oblique sheet of shell-bearing alluvi- 

 um; and that the level of the lake has correspondingly fallen. 

 Lake Nicaragua is to-day more than one hundred feet above 

 sea-level. The San Juan River therefore has sufficient fall 

 for further effective erosion, which is, without doubt, going 

 on at an increasing rate, and the conclusion seems irresistible 

 that the lake is destined to a further reduction of its level, and 

 a still further contraction of its area, and that within no distant 

 future; since it is evident from the comparatively perfect pres- 

 ervation of the shells above mentioned that the whole shell- 

 bank is a very recent affair indeed, as terrestrial changes go. 



In view of the attempted construction of vast public works 

 along the valley of the San Juan River, the importance of this 

 conclusion is sufficiently obvious. 



While the lake cannot be wholly drained, its greatest depth 

 being about two hundred and forty feet and the elevation of 

 its surface above sea level beiag one hundred and ten feet, its 

 average depth is such that a comparatively slight fall in its 

 general level must cause a great contraction of its area, — a 

 circumstance of much importance to the cities and towns along 

 its shores. 



The building of the proposed great Ochoa dam across the 

 lower San Juan River in connection with the construction of 

 the Nicaragua Canal is probably the only thing that would 

 arrest this erosion, and would secure the retention of this vast 

 and important inland reservoir of fresh water. 



