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some of these valleys their thickness cannot fall far short of four or five hundred feet. At the Artesian Well, a few miles west of Fort Fillmore, they were penetrated to the depth of two hundred and fifty-eight feet without reaching their base, and a little farther north, at the San Diego Mountain, vertical sections are exposed to the height of near five hundred feet. (Vide Report on the Jornada del Muerto.)

Economic Uses.-Should it ever become necessary to construct macadamized roads over any portion of the region in which the boulder formation occurs, it would furnish excellent materials for that purpose. The limestone boulders would also form a good quicklime, and the sand would be well adapted for mortar.

We have been able to detect strata of the Tertiary Period only in two localities throughout the entire district under examination, and even in these the formation is developed upon comparatively an insignificant scale, extending over very limited areas, and attaining a vertical thickness of merely a few feet. Both localities where we have recognized it are in Southern Texas. One is situated thirty-five miles southeast of San Antonio, and was crossed by the Expedition on the 9th and 10th of April, 1855. (Vide Journal.) The Tertiary Strata present here a thickness of about twenty feet, and repose unconformably partly upon the sandstones and grits of the Coal Measures and partly upon the marls of the Cretaceous System. The rock is a dirty ferruginous limestone of a deep yellow color, and varies much in compactness, some of the layers being quite soft and crumbling, and others hard and breaking with an uneven fracture. It contains fossils in great abundance ; but they are usually casts, and so badly preserved as not to permit us to determine accurately their specific characters. They belong mostly to the genera Tellina, Arca, Infundibulum, Fusus, and Natica.

The next locality at which Tertiary Strata were observed is on Leon Creek, a few miles west of San Antonio. Here it consists of light gray earthy limestone, which is scattered at several points over the surface.

Although we have not seen Tertiary Strata in other localities than those just mentioned, there is reason to believe that they are of extensive occurrence in Eastern and Northeastern Texas. Dr. F. Roemer mentions the occurrence of this formation on the Brazos River near the town of Caldwell, and also in the vicinity of Nacogdoches. Assuming this last point as the extreme northeastern limit, it would give to this formation in Texas a northeast and southwest range of about three hundred miles. Nevertheless, as extensive as are these limits, we are of opinion that they include only a portion of the Tertiary area of this region, and that more extended research will develop its existence as far south as the Rio Grande.