FLUORINE IN WATER SUPPLIES—VAN BURKALOW 219 
pyrite are abundant. Because for part of this area, North and South 
Dakota, fairly complete information was available on both the fluorine 
content of the water supplies and the aquifers from which these sup- 
plies are derived, the Dakotas were chosen as a sample area to demon- 
strate methods and problems of mapping these factors (figs. 2 and 3). 
Examples in this region of formations containing pyroclastic material 
and pyrite associated with lignite, and also yielding water with a 
fairly high fluorine concentration, are the Benton and the Pierre. 
The Fox Hills sandstone contains volcanic ash and yields water fairly 
rich in fluorine but is not described as containing lignite or pyrite. 
On the other hand, the Lance and Fort Union formations, which in 
western North Dakota are the chief sources of fluorine-rich waters, 
contain much lignite with a variable sulfur content but are not re- 
ported to contain volcanic materials. Over large areas in the Great 
Plains artesian water from the Dakota sandstone is the chief, and 
sometimes the only, dependable source of water, but most of the 
samples analyzed showed toxic amounts of fluorine, enough to cause 
severe forms of mottled enamel. Even before the cause of mottled 
enamel had been determined, it was known that in several areas this 
malady had appeared simultaneously with the installation of artesian 
wells tapping the Dakota. There are frequent occurrences of lignite 
in this formation, sometimes with notable concentrations of pyrite, 
but only a rather remote relation to volcanic activity can be demon- 
strated. 
No mention can be found in the literature of volcanic materials in the 
Dakota sandstone within the United States. In the eastern Rocky 
Mountains of Alberta, however, a thin bed of tuff near the middle of 
the formation has been described, and the upper part contains tuf- 
faceous material grading into the overlying Crowsnest volcanics,” 
which were derived from several vents in that area. Since volcanic 
activity is known to have taken place to the northwest in Alberta and 
to the south in Arkansas, Oklahoma, and Texas (the Woodbine is 
correlated with the Dakota), it seems possible that there were other 
volcanic vents that could have furnished fragmental material to the 
northern and central Great Plains. Some of the dark shaly layers 
within the Dakota may prove to be volcanic in origin. On the other 
24 Darton, N. H., Preliminary report on the geology and underground water resources of 
the central Great Plains, U. S. Geol. Surv. Prof. Pap. 32, 1905; Rubey, W. W., Lithologic 
studies of fine-grained Upper Cretaceous sedimentary rocks of the Black Hills region, ibid., 
165—A, 1931; Spivey, R. C., Bentonite in southwestern South Dakota, South Dakota State 
Geol. Surv. Rep. Investigations No. 36, 1940; Stanton, T. W., A Cretaceous volcanic ash 
bed on the Great Plains in North Dakota, Journ. Washington Acad. Sci., vol. 7, pp. 80-81, 
1917; Wherry, E. T., Clay derived from voleanie dust in the Pierre in South Dakota, ibid., 
pp. 576-583; Wing, M. E., Bentonites of the Belle Fourche district, South Dakota State 
Geol. Surv. Rep. Investigations No. 35, 1940. 
* MacKenzie, J. D., The Crowsnest volcanics, [Nat.] Mus. [Canada] Bull. No. 4 (Geol. 
Ser. No. 20), Canada Geological Survey, 1914. 
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