396 The American Geologist. December, 1901 
river valley. It was filled with ice to the prairie level. When the 
ice melted and the higher land appeared, the water cut new channels 
along the border in many places ; notably at the upper and lower 
rapids, and further down. The "old father of waters," has not had 
sufficient time or force to deepen these channels. It is on these in- 
complete cuttings that the United States government is spending large 
sums of money. At the closing up of the ice-period many of the 
tributary streams also lost their old courses and cut new ones. This is 
the situation of many of the rivers of Iowa which has puzzled ob- 
servers. P. J. Farnsworth. 
Clinton, lozva, Oct. 12, igoi. 
Recent Decline in the Level of Lake Nicaragua. — About a year 
ago (October, 1900), Mr. J. Crawford published a letter in the American 
Geologist in which criticism is made of Dr. Heilprin's claim that the 
water-level in lake Nicaragua had fallen 20 feet between the years 1880 
and 1898. In the course of his criticism Mr. Crawford says : "If be- 
tween 1880 and 1898 there has been a decline of 20 feet from the 1880 
level of lake Nicaragua from any cause, then the beach marks of the 
1880 level would be easily found along the borders of the lake. But 
there is no such evidence to be found." While I have no data which 
would enable me to discuss the time-element in controversy, I did secure 
evidence, during my visit to Nicaragua in 1892-3, of a comparatively 
recent subsidence of the level of lake Nicaragua, and discussed it briefly 
in a paper published in 1896.* 
It may be of interest, even at this late day, to quote the papc^r in 
question : 
"The peculiar exposure of which notice is here made is found on the 
Manuel Vargas ranch, east of San Carlos, Nicaragua, on the south bank 
of the San Juan iriver. The river, here only twelve miles from the cutlet 
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 fif- 
teen 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 packd 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 11 om 
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 fiftee 1 or 
twenty feet above the river which had then (March 12th, 1893) 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 collected 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 shel]> "^lill 
*BuII. Lab. Nat. Hist., state Univ., Iowa. vo\.iv. pp. " <-95. \ M.m-. m 
Sheil-bank. 
