walcott.) IOWA. 187 



The writer -wishes to correct the view published by him in the American Geologist 

 a few years ago ("A Great Primordial Quartzyte," vol. 1, p. 178), that the granular 

 quartz probably overlies the black slate of the Taconic system, and is equivalent of 

 the Red sandrock of Vermont. The Red sandrock appears to be a part and near the 

 top of the "Georgia series" and above the Winooski marble, but the "granular 

 quartz" lie's below the Winooski marble. The typical Potsdam is probably the 

 " granular quartz." But the Red sandrock, and also some light-colored, loose sand- 

 stones still higher have very largely been regarded its equivalent. 



IOWA. 



On the map accompanying Dr. D. D. Owen's report of the geological 

 survey of Wisconsin, Iowa, and Minnesota a small area is colored as the 

 "Potsdam sandstone of New York, iu the northeastern corner of the 

 State. 1 



As State geologist of Iowa, Prof. J. Hall stated that the Potsdam- 

 sandstone attains its greatest exposures in Minnesota and Wisconsin 

 north of the limits of Iowa, and about the regions of Lake Fepin. The 

 excavation of the Upper Iowa River, however, has removed the cal- 

 ciferous sandstone ; so that at the junction of that river with the Missis- 

 sippi there is exposed a broad belt of the lower rock. A general de- 

 scription of the sandstone is given with the statement that some slightly 

 calcareous bands contain fragments of trilobites. In numerous locali- 

 ties shells of Lingula are found, though by no means so abundantly as 

 in the same rocks in Minnesota. These fossiliferous bands appear in 

 the vicinity of Lansing. 2 In the same volume there is a general ac 

 count of the Potsdam or inferior sandstone by Prof. J. D. Whitney. 3 



In a general description of the geology of Iowa Dr. O. A. White de- 

 scribes the Sioux quartzite as occurring in the extreme northwestern 

 corner of the State. In the table of classification he places it as Hu- 

 ronian (*?) and beneath the Potsdam sandstone. 4 This is the quartzite 

 referred to the Potsdam in Minnesota by Prof. N. H. Wincuell. In 

 describing the Potsdam sandstone Dr. White states that it reaches a 

 known thickness in Iowa of 300 feet, and it is exposed only in a small 

 portion of the northeastern corner of the State. 5 

 • In the account of a deep well at Emmetsburg, Iowa, Prof. ST. H. Win- 

 ch ell refers to a white sandstone found beneath the blue shales of St. 

 Croix, assigning it a thickness of 107 feet. The rock beneath this, which 

 Mr. Swan, who kept a record of the well, called granite, is referred to 

 the Potsdam quartzite. 6 



1 6eol. Surv. of "Wise, Iowa, and Minn.; and, incidentally, of a portion of Nebraska Territory, 

 Philadelphia, 1852. 



'Geology of Iowa, vol. 1, 1868, pp. 47-48. 



8 Ibid., pp. 328-331. 



4 General geology. Azoic, Lower Silurian, Upper Silurian, and Devonian systems. Geol. Surv. Iowa, 

 Rep., vol. 1, 1870, p. 168. 



6 Op. cit,,p.l72. 



6 Section of a deep well at Emmetsburg, Iowa. Minn. Acad. Soi. Bull., voL 1, 1880, pp. 387, 388. 



