Correspondence. 397 
have their valves united by the ligament, and not a few are in a vertical 
position, showing that the shells had been deposited in situ. These 
species are now found living in abundance in the river in the mud close 
to the shores, and the fossil shells no doubt were developed under s-im- 
ilar 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 alluvium ; and that the level of the lake has correspondingly 
fallen. Lake Nicaragua is today more than one hundred feet above sea- 
level. The San Juan river therefore has sufficient fall for further ef- 
fective 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 compara- 
tively perfect preservation 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 being one hundred and ten feet, its average depth is such that 
a comparatively slight fall in its general level must cause a great con- 
traction 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 retenton of this vast and important inland reservoir 
of fresh water." 
All that part of the river valley lying above the point here discussed 
was evidently at no remote time covered by the waters of the lake which 
have since receded to San Carlos because of the lowering of the river 
channel and consequently of the lake level. 
The deposit of shells here discussed is not to be compared with the 
shell heaps sometimes found along shores, but was evidently formed 
gradually, and represents an old Unio bed. 
In considering the extent of the change in the altitude of the writer- 
level in the lake, which is shown by this deposit, it must be borne in mind 
that to live Unios must be constantly submerged, and that therefore the 
highest portion of the shell-bank represents what at one time was less 
than the lowest stage of water in the lake. It is true that these Unios 
(fresh water mussels) are capable of creeping about more or less, but 
nowhere do whole colonies of them move upward with the floods, to be 
stranded when the waters again recede. 
