20

The inferior division of the Cretaceous System is composed chiefly of marly clay, sandstone, and gypsum. The clay and sandstone occur in alternating beds of variable thickness, and constitute the chief mass of the formation; the gypsum usually occurs towards the top, and is either found upon the surface or interstratified with the clay in nearly horizontal bands.

The Marly Clay varies considerably in lithological character, and, as the name implies, it is composed mainly of clay and carbonate of lime combined in different proportions, with which there is frequently a large admixture of sand and gypsum. The prevailing color is a deep venetian red, often elegantly variegated with various shades of blue, brown, and yellow. It is for the most part highly indurated, and sometimes exhibits a schistose structure not unlike the shales of the Coal Measures. In some localities, particularly towards the western boundary, it abounds in small rounded pebbles of granite, porphyry, quartz, and other eruptive rocks. Near the upper part the layers are often beautifully reticulated with gypsum.

The Sandstone passes from a fine-grained variety to coarse gritstone, and its colors are dark purple, red, brown, yellow, gray, and white. In many instances it is thickly marked with small circular light yellow and greenish spots, which are distributed through all the beds. The layers are often finely laminated, and sometimes contain rounded pebbles of eruptive rocks and nodular concretions of iron. The sandstone often contains a good deal of calcareous matter, and sometimes passes gradually into limestone. Usually it is quite soft, being in many instances but little more than slightly coherent sand, which crumbles more or less rapidly when exposed to the weather.

The Gypsum is mostly white, amorphous, and occurs in layers, sometimes of enormous thickness. This rock will be more particularly described under the head of useful minerals.

The Marly Clay Group attains a vertical development of several thousand feet, and as far as our observations have extended, forms, with a few local exceptions, the prevailing formation of all that portion of the Plains east of the Rocky Mountains not included within the limits of the superior division of the Cretaceous Group. It is exposed at various points subordinate to