GEOLOGICAL PAPERS 109 



in Rock Island county, above the level of the Phillipsastrea bed, 

 are lithologically and faimally similar to the Cedar Valley strata 

 of Iowa, and may be correlated with certainty with the rocks of 

 that stage. 



According to Hutton's unpublished thesis, the limestones of 

 Jersey and Calhoun counties, Illinois, are of the same age as 

 those of the Cedar \'alley stage, in Iowa, lying between the 

 Stromatopora reef and the Acervularia dazndsoni zone. Conse- 

 quently they may be correlated with the equivalent portion of 

 the Rock Island county- section. The upper portion of the Rock 

 Island county strata may likewise be correlated with the Hamil- 

 ton limestone of northern Missouri, which corresponds in time 

 with the rocks of the Cedar \'alley stage in Iowa. 



The Rock Island count}* Devonian limestones are a continua- 

 tion eastward of the Wapsipinicon and Cedar Valley stages of 

 Iowa. They represent the same general period of time and the 

 same general conditions of deposition. 



A TUFA DEPOSIT XEAR DAXVILLE, ILLIXOIS. 

 Charles E. Decker. 



While conducting an excursion from the University of Illinois, 

 last summer, along the \*ermilion River, in the vicinit}' of Dan- 

 ville, some large fragments of tufa were discovered near the base 

 of a slope. Search for the origin of these fragments led to the 

 discovery of an extensive tufa deposit farther up the hill. This 

 deposit is on the farm of Mrs. Mar\' A. Kistler, on the north side 

 of the Vermilion River, one-half mile northeast of the confluence 

 of the ^Middle and Salt Forks, and one and one-half miles south- 

 west of Hillery. The upper part of the tufa borders on the lower 

 edge of a spring basin (Fig. i) in which the spring issues sixty 

 feet back from this edge. 



The tufa is of special interest, because no other work of this 

 kind by ground-water is known in this region. Solution on a 

 large scale is shown by the extensive caves of southwestern 

 Indiana, but the nearest tufa deposits known to the writer are 

 those noted by Grant and Burchard^ in southwestern Wisconsin 

 along the Platte, Little Platte. Grant and ^lississippi Rivers. 



1 Lancaster-Mineial Point Folia, p. 9. 



