256 FIELD CoLUMBIAN MusEUM—GEOLOGY, VOL. 1. 
Data are meagre for estimating the length of this period. Prestwich* 
has estimated the rate of erosion by the Thames as one inch in 1,000 
years. The chalky Cretaceous and Odlitic strata over which the 
Thames flows are doubtless eroded at a more rapid rate than the com- 
pact limestone in which Wyandotte cave is situated. Taking this 
rate, however, as a minimum, it will be found that a period of 360,000 
years would be required to erode the ‘‘Senate Chamber” to the 
depth of the base of the stalagmite. 
MARENGO CAVE. 
THE CAVE FLooR TerRRACE.—The greater portion of the floor 
of this main cave shows a well marked terrace recording two distinct 
stages in the life of the stream which before its final disappearance 
flowed through the cave. Of these two stages the stream of the 
older stage had a width of from 15 to 20 feet and a current of 
sufficient velocity to make large ripple marks on its bed of coarse 
alluvium. These ripple marks are symmetrical and their long 
slope is plainly away from the present entrance to the cave. This, 
therefore, was the direction of flow of the stream. In its second 
stage the stream was reduced to a width of about 10 feet and its cur- 
rent was more sluggish. It cut a trench of the above width with 
nearly vertical walls to a depth of about two feet in the bed of the old 
stream, but did not have a current of sufficient velocity to produce 
ripple marks on its bed. A further greater sluggishness as compared 
with the first stream is indicated by the somewhat winding course 
which it took through the bed of the latter. The disappearance of 
this stream must have taken place somewhat suddenly, for there has 
been no trenching of its bed nor sloping of its banks such as would 
have occurred if the flow of water had diminished gradually. The 
bed, now quite dry, has a slightly concave form. A draining away of 
the stream by the opening of new conduits at a lower level seems the 
most natural explanation of its two stages and final disappearance. 
It is of course not impossible that these stages mark a diminution in 
rainfall or supply of waters from above, but there is on the whole 
little reason to suspect such abrupt changes in these conditions. It 
is not unlikely that the large spring situated a few rods west of the 
present entrance represents the present point of issue of the stream. 
*Geology, vol. I, p. 107. 
