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METALLIC ORES.

The only ores of this formation of sufficient importance to require notice are those of iron. This metal is disseminated through the different layers, principally in the form of oxides and the bisulphuret or iron pyrites.

Oxides of Iron.-The coloring matter of most of the limestones and sandstones is due to the oxides of iron, which also exist in the form of spherical concretions, which are sometimes so abundant as to impart to the rock a highly mottled appearance. Brown Hematite of very fair quality is found in beds of considerable thickness in connexion with these rocks along the Upper Brazos River.

Bisulphuret of Iron (Iron Pyrites), in cylindrical and globular masses, often studded with beautiful crystals, are of frequent occurrence in the limestone.

AGRICULTURAL CHARACTER.

By far the largest portion of the rich agricultural district of Texas is comprised within the limits of this formation. Whenever favorably situated for natural irrigation the soil derived from these rocks is usually highly productive and well adapted-for the growth of the great staples, corn, cotton, wheat, tobacco, and almost every product suited to the climate. But as we have already spoken of this subject in detail in connexion with the quaternary deposits and in the Journal, it is unnecessary to pursue it any farther at present.

SPRINGS.

Although not generally so abundant as we might be led to suppose from the character of the rocks, springs are nevertheless of frequent occurrence in various portions of the region occupied by this formation. Indeed in some instances they furnish almost the only supply of water over districts of country hundreds of miles in extent. They are sometimes unusually large, and generally much more abundant near the base than towards the summit of the formation. The springs in the vicinity of San Antonio and Fort Washita issue from near the junction of the limestone with the underlying marly clay, and afford streams of considerable magnitude. The San Antonio, Guadalupe, Frio, Seco, Medina, San Pedro, Concho, San Saba, False Washita, Brazos, and Red Rivers all originate from springs that gush from the Upper Cretaceous strata.

Subterranean Rivers are known to occur in connexion with these rocks in several parts of Texas. The most remarkable one known to us was discovered near the northern base of the Limpea Mountains by Mr. Thompson, a member of your Expedition. This stream emerges from a triangular