no ILLINOIS AC.\DEMY OF SCIENCE. 



In form the deposit resembles an alluvial fan. It extends down 

 the hill about 150 feet and widens from a few feet across at the 

 top to about 150 feet in width at the base. Accordingly, its 

 approximate areal extent is 10,000 square feet. The thickness 

 may be seen along- its eastern edge, where it is exposed in the 

 side of a gully throughout its entire length. Here the thickness 

 varies from two to four feet. If an average thickness of three 

 feet be taken for the entire deposit, it comprises about 30,000 

 cubic feet. 



The composition is almost pure calcium carbonate (CaCOs). 

 It contains little extraneous material except along the base, where 

 it filled in the spaces in the underlying glacial gravels, thus includ- 

 ing some of them in its lower part. 



The included fossils comprise leaves and shells. The leaves are 

 large with open venation and resemble those of the Sycamore or 

 Sugar Maple, which are the oldest trees now growing in the 

 vicinity. Locally, the leaves are so abundant that a rude cleavage 

 is developed in the tufa parallel to their flat faces. The shells are 

 all gastropods with a spire of medium height. Those of a single 

 species, Polygyra clevata," say, were scattered in great numbers 

 over the surface of the deposit, and a single specimen was found 

 within it. 



The texture of the tufa varies from porous and open to com- 

 pact. In some parts it shows a botryoidal or mammilary struc- 

 ture, while in others a delicately branching, moss-like structure 

 may be seen. In still other parts a cleavage is developed, as noted 

 above, due to the presence of numerous fossil leaves. 



The surface of the tufa is mostly covered by a fine calcareous 

 dust, though some larger fragments of the material lie scattered 

 about. As indicated by the dust, the surface is devoid of smaller 

 vegetation, but it is sparsely covered with bushy trees of Black 

 Locust, Hawthorn, Elm, Willow and Apple. 



The topographic relation of the tufa to the spring suggests that 

 it was deposited by the water issuing above it. However, these 

 waters are not now depositing, but, instead, are degrading the 

 earlier deposit. The spring flows at the rate of three-fourths of 

 a gallon per minute. This water and that collected by the spring 

 basin has cut a gully fifteen feet deep in the eastern edge of the 

 tufa, and has carried much of it away. The west side of this 

 gully is so white with calcareous dust and fragments of the tufa 



Identification by J. D. Hood. 



