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The next formation that we meet, in descending order, in the district under consideration, is the Cretaceous System, which, on account of its great development upon the Western Plains, is far more important than any we have described. The strata of this system here exhibit the enormous thickness of three thousand six hundred feet, and occupy an area of many thousand square miles. With some local exceptions, they cover nearly the whole of the rectangular space included between the 29th and 39th degrees of latitude and the 99th and 105th degrees of longitude, extending southeast into the States of Louisiana, Mississippi, and Alabama, and north into the Territories of Kansas and Nebraska.

In the southern part of the district we have traced them almost continuously from near Victoria on the Guadalupe River in a course nearly W. N. W. to within a few miles of the southern extremity of the Guadalupe Mountains, and thence southward along the base of the Limpea Mountains. Farther north we have encountered them at Fort Towson, Fort Washita, Preston, the Cross Timbers of Texas, and throughout the entire country watered by the Big and Little Witchita, and the upper portions of the Trinity, Brazos, and Red Rivers. Along the Canadian River they have been met with by Mr. Marcon, and north of that stream by Maj. Emory, Lieut. Simpson, Lieut. Abert, Capt. Stansbury, and other explorers.

In the Horse Mountains, and at several other points west of the Guadalupe Mountains, the Cretaceous rocks have likewise been encountered, but we have met with no proofs of the existence of deposits of this age in any portion of the region explored by ourself west of the Rio Grande. North of this, however, along the 35th parallel of latitude, they have been observed by Mr. Jules Marcon, and from the explorations of others we have abundant evidence of their existence in detached basins as far west as the Pacific coast. Dr. John Evans, U. S. Geologist, has recognized well marked Cretaceous strata at a number of points in Oregon and as far west as Vancouvers Island.

The Cretaceous Formation of our district may be separated by well marked lithological characters into two principal groups, viz.:

• 1. Tipper Cretaceous or Calcareous Group.
• 2. Lower Cretaceous or Marly Clay Group.

These rocks have been described by Mr. Jules Marcon as belonging mostly to the Triassic and Jurassic Systems, but evidence of their Cretaceous age will be presented farther on.