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the last, and is well ascertained to occur as far east as Fort Washita, in the Chickasaw Territory.

Beginning in the southern part of the district under notice, we find this formation well developed along the route traveled by your Expedition between Victoria and San Antonio, Texas, where it is frequently exposed in natural sections from fifty to a hundred feet high. Near San Antonio it has been artificially excavated in one place to the depth of a hundred and ten and in another to a hundred and fifty feet without reaching the base. Immediately west of this it disappears beneath the rocks of the Upper Cretaceous Group, and does not again show itself in that direction, save at a few isolated points, until we arrive at the abrupt escarpments known as the Castle Mountains," near the base of which it is seen reposing in nearly horizontal layers (vide Journal), thence westward it forms, with the exceptions pointed out in the preceding chapter, the only rocks to be seen until we approach the sources of Delaware Creek, where it is succeeded by the Paleozoic rocks.

West of the Guadalupe Mountains we again observe the Marly Clay Group, but here it is developed upon a very limited scale, being only a few feet in thickness, and confined to a strip of country not exceeding fifteen or twenty miles in width. West of this we have not been able to recognize positively its existence over any portion of the region explored by ourselves, but from the examinations of others it is highly probable that it appears in detached basins over a large portion of the region comprising the Western or Pacific Slope of our continent.

Farther north we have ascertained its existence at Fort Washita, at several points between there and the Upper Cross Timbers of Texas, and throughout the greater portion of the region watered by the Big and Little Witchita, Upper Brazos, and Red Rivers, and still farther north it has been encountered by Capt. Marcy, Lieut. Simpson, Mr. Marcon, Col. Long, and numerous other explorers.

Thickness.-As deep natural sections are very rarely observed in this formation, we can merely estimate its thickness approximately. As already stated, it has been artificially excavated in the vicinity of San Antonio to the depth of a hundred and fifty feet without reaching the base. Near Fort Washita it is exposed in natural sections sixty or seventy feet high. At the first Artesian Well, located upon the Llano Estacado, about fourteen miles east of the Rio Pecos, the Marly Clay formation was penetrated to the depth of six hundred and forty-one feet, and at the second one, located a few miles farther south, the borings were carried to the depth of eight hundred and fifty-eight feet, and in neither instance was the base of the formation reached. On the Big and Little Witchita and Brazos Rivers it forms bluffs from two to five hundred feet high, while. along the Upper Red River its general thickness, estimated after an examination of numerous sections, cannot be less than two thousand five hundred feet. This is