28

Mountains, with an average width east of the Rocky Mountains of perhaps not less than two hundred and fifty miles.

West of the Rio Grande gypsum is also met with in great abundance, and towards the south we have abundant evidence that it extends through various portions of Mexico. As already remarked, it occurs chiefly towards the summit of the Marly Clay formation, and is found either upon the surface or interstratified with the clay in successive and nearly horizontal layers of variable thickness. It usually occurs amorphous and pure white, more or less granular, and sometimes its texture is subcrystalline, resembling loaf sugar. Not unfrequently it passes into selenite, fibrous gypsum, or compact alabaster. In some localities it is much discolored with oxide of iron, in others it is variegated with various shades of green and blue by oxides and carbonates of copper. Between the beds the clay is often thickly reticulated with thin veins of selenite and fibrous gypsum.

The thickness of the different beds is sometimes enormous. On Red River they vary from a few inches to twenty-five or thirty feet. On Delaware Creek, a few miles below its source, bluffs of pure white gypsum are exposed to the thickness of about sixty feet, while between the Big Witchita and Brazos Rivers hills composed entirely of gypsum were encountered, whose heights were estimated by myself, as well as by several others of the party, at about seven hundred feet.

We have detected in this formation ores of Iron, Copper, and Manganese. Of these the most abundantly distributed are those of iron, which, in the form of oxides, furnish the principal portion of the coloring matter in the clays and sandstones.

Brown Iron Ore (Brown Hemitite) occurs in workable quantities in several localities along the Big Witchita River. It sometimes contains a large admixture of clay. In the form of Yellow Ocher, it is often used by the Indians as a paint.

Bisulphuret of Iron (Iron Pyrites) occurs in small crystals and spheroidal masses, occasionally disseminated through the clay. The spheroidal form is often beautifully studded with minute shining crystals, and when broken usually exhibits a fibrous radiated structure.

Ferruginous Sands of remarkable purity occur in great abundance on Cache Creek, an affluent of Red River. Specimens which we collected from this locality were submitted to Prof. C. U. Shephard for examination, and found to be-composed of about equal proportions of tatiniferous and magnetic oxide of iron. These sands have doubtless been derived from the Witchita Mountains.

Marcy's Report of Red River of Louisiana.