216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
observed in individual rocks. For example, the granites of the Pikes 
Peak area contain from 0.04 to 1.00 percent of fluorine, present in the 
form of workable veins and scattered crystals of fluorite and of rare 
fluorine-bearing minerals.’ As a result all the surface streams drain- 
ing the area contain more than 1.0 p. p. m. of fluorine, enough to make 
mottled enamel endemic in the Colorado towns deriving their water 
supplies from these streams. This is one of the few cases of endemic 
mottled enamel! associated with the use of surface waters. 
Very few sedimentary rocks have been tested for fluorine. How- 
ever, since most underground water comes from sediments, it is evident 
from this study that such rocks differ greatly in fluorine content. 
In some regions some of the strata contain a considerable amount of 
fluorine, others very little. Even in one stratum the amount varies 
from place to place. What is the source of this fluorine in sedimentary 
rocks? Obviously, some of it is furnished by the weathering and ero- 
sion of the fluorine-bearing minerals in igneous rocks, the commoner 
of which are apatite, tourmaline, topaz, and some of the micas. In some 
regions fluorine has been added in solutions directly from magmatic 
sources. This is probably the origin of the fluorspar deposits of Ken- 
tucky and Illinois.** Yet another source, varying in importance from 
time to time, is the exhalation into the atmosphere of fluorine gases 
from volcanoes. Zies?® has shown that in the Katmai area alone an 
estimated 135,000 metric tons of fluorine is released yearly, in the form 
cf gaseous hydrofluoric acid. This material, dissolved in rain or 
snow,’* is washed out of the atmosphere and contributed to the ground 
water or the ocean or to sediments forming at the time. Mansfield 
has suggested that because phosphates and fluorine have an affinity 
for each other, deposition of such fluorine would tend to be localized 
in areas where conditions favored the accumulation of phosphates. 
The resulting fluorapatite is less soluble than the original “bone phos- 
phate” and can be preserved for a long time. Mansfield believes, there- 
tore, that only during times of marked volcanic activity, when large 
amounts of fluorine are being supplied, can large enduring deposits of 
3 Clarke, F. W., Analyses of rocks and minerals . . ., 1880 to 1914, DU. S. Geol. Surv. Bull. 
591, pp. 118, 290, and 291, 1915. 
14 Currier, L. W., Fluorspar deposits of Kentucky, Kentucky Geol. Surv., ser. 6, vol. 13, 
1923; Bastin, E. S., The fluorspar deposits of Hardin and Pope Counties, Il]., [linois State 
Geol. Surv. Bull. No. 58, 1931. 
4% Zies, H. G., The Valley of Ten Thousand Smokes, Nat. Geogr. Soc. Contr. Techn. Pap., 
Katmai ser., vol. 1, No. 4, 1929. 
10Tt is therefore possible for rain water to contain small amounts of fluorine before it 
touches the ground. W. H. MacIntire, W. H. Shaw, and B. Robinson (A barium-fluorine 
study: The fate of added barium silicofluoride and its effect upon sulfates and other soil 
components, as influenced by limestone and by dolomite, Tennessee Agr. Exp. Stat. Bull. 
No. 155, 1935) found that the rainfall at Knoxville, Tenn., for 1 year, from July 1933 to 
July 1934, contained 0.145 p. p. m. of fluorine. They suggest that most of it came from 
the smoke and soot from bituminous coal. 
1 Mansfield, G. R., The role of fluorine in phosphate deposition, Amer. Journ. Sci., vol. 
238, pp. 863-879, 1940. 
