IOWA CAMBRIC SUCCESSION 
317 
portion of these sediments belong to Early Cambric age. Whether 
any part of the basal Mid Cambric series is present remains to be 
determined. The so-called Tipton sandstones and shales^® may 
eventually prove to be Mid Cambric in age rather pre-Cambrian 
to which they are now referred. 
To the Croixan series, of Mid Cambric affinities, which com¬ 
prises the lowermost succession of sandstones, Owen^^ and other 
early geologists in this region gave the designation Lower Sand¬ 
stone. In after years this sequence was correlated with the Pots¬ 
dam sandstone of the New York section. White thus makes 
allusion to it. Even so late as 1907 Calvin is inclined to this old 
mis-fit nomenclature. 
The eroded surface of ancient crystallines upon which these 
Croixan sandstones recline is distinctly an old peneplain, as 
smooth and as extensive perhaps as ever chances to develop. Like 
the great basal sandstone of the Coal Measures the bottom ter- 
rane of the Cambric section of the region probably is not in its 
several parts of the same age, but constitutes a homeogeneous 
lithologic unit disposed obliquely to the general stratigraphic 
planes. 
To this basal arenaceous member the Minnesota title Hinckley 
Sandstone should be restricted, if that name is to be used at all. 
In the reports of the Iowa Survey the loose significance of the 
northern workers is followed. Now, since this name is severely 
limited it should be so retained until it is finally demonstrated 
that some other title is more appropriate. There is possibility of 
this, because as yet there are no published data, according to 
Upham indicating that at the original locality in Pine County, 
Minnesota, the Hinckley Sandstone is the lowermost Paleozoic 
formation. 
Although the Hinckley Sandstone, as thus defined, finds no 
outcrop within the limits of Iowa, it is easily recognizable in 
deep-well logs. In the Tipton well section as analyzed by 
16 Proc. Iowa Acad. Sci., Vol. XIX, p. 149, 1912. 
17 Geol, Surv, Wisconsin, Iowa and Minnesota, p. 48, 1852. 
18 Geology of Iowa, Vol. I, p. 171, 1870. 
19 Minnesota Geol. Surv., Final Rept. Vol. II, p. xxii, 1888. 
20 Iowa Geol. Surv., Vol., I, p. 23, 1892; also. Ibid,, Vol. XXII, p. 155, 1913. 
21 Minnesota Geol. Surv., Final Kept., Vol. II, p. 629, 1884. 
22 Iowa Geol. Surv., Vol. XXI, p. 215, 1912. 
