Note 18 — THE CHEMICAL RELATION OF IRON AND MANGANESE IN SEDIMENTARY ROCKS.

The solutions may be precipitated, as already shown, either with or without admixture with mechanical sediments; and there are in nature all gradations from almost pure deposits of iron and manganese ore to beds of shale, sandstone, etc. stained with iron or manganese. Subsequent concentration frequently causes decided changes in the latter deposits (see p. 370).