A NlCARAGUAN SHELL-BANK. 



By B. SHIMEK. 



The peculiar exposure of which notice is here made is 

 found on the Manuel Vargas ranch, east of San Carlos, Nica- 

 ragua, on the south bank of the San Juan River. The river, 

 here only twelve miles from the outlet of Lake Nicaragua, 

 flows due east and cuts into a terrace on the southern shore, 

 forming a vertical bank which gradually rises from the low 

 river-bottom at its eastern extremity, and reaches a height of 

 more than fifteen feet at the western extremity of the shell- 

 bearing portion. This bank, for one hundred yards from its 

 eastern extremity, consists of fine alluvium which is literally 

 packed with Unio shells, frequently cemented together by 

 calcareous tufa into large masses. 



The shell-bearing portion of the exposure averages about 

 twelve feet in height, but the surface of the ground gradually 

 rises back from the river, and shells were traced along the 

 surface to a point more than one hundred and fifty feet from 

 the shore, this point being fifteen or twenty feet above the 

 river which had then (March 12th, 1892) not yet reached its 

 lowest stage. This indicates a total thickness of the deposit 

 of more than fifteen, and probably not less than twenty feet. 



There are three species of Unio in the series of shells col- 

 lected from the bank, and they are identical with living species 

 found now in the mud of the river at the foot of the exposure. 

 Many of the fossil shells still 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 



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