Note 13 — THE CHEMICAL RELATION OF IRON AND MANGANESE IN SEDIMENTARY ROCKS.
Reading edition
The exact age of the iron and manganese deposits here referred to is, in some cases, a little uncertain. Some may be Cambrian, others Silurian, but the exact determination of the age of the horizon is not a part of the present discussion. The matter has been discussed by the writer in Geological Survey of Arkansas, 1890, Vol. I., pp. 376–379.