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ORIGIN.

The materials composing the larger portion of the Marly Clay formation have been deposited in the bottom of the ocean in the condition of fine sediment, which appears to have been derived chiefly from the destruction of the granites and porphyries that we find towards the western slope of our continent. These are generally of a deep red color, and their disintegration produces materials similar to those constituting the larger part of the Marly Clay formation. The great thickness and extent of this group, covering, as it does, many thousands of square miles, seems to preclude the idea of its having been derived from the destruction of the rocks of the Plains east of the Rocky Mountains, since granites and porphyries that would produce such a deposit are there of uncommon occurrence. Moreover, we find in the marly clay and sandstone, pebbles identical in composition and appearance with the granites and porphyries that form the eruptive ranges towards the western side of the Continent. These pebbles also increase in size and number as we advance in that direction, and the formation itself is greatly increased in thickness. At Fort Washita it is probably not more than a hundred feet thick, while farther westward it is estimated to be not less than twenty-five hundred feet thick.

PALEONTOLOGY.

This formation has been described by Mr. Marcon as belonging partly to the Jurassic and partly to the Triassic Systems. Under the Jurassic he includes a large proportion of the strata described in the preceding chapter, while the Triassic is made to occupy, with a few local exceptions, not only all that portion of the Plains east of the Rocky Mountains not included within the limits of his Jurassic, but also a very extensive area in other parts of the United States.

The data for these conclusions appear to have been derived mainly from the region along the Canadian River, explored by Lieut. Whipple's party, of which Mr. Marcon was a member. Our observations on the geology of the Upper Red River country have also been brought forward in support of these views. Inasmuch, however, as we cannot agree with this geologist in his opinions respecting the parallelism of these strata, we shall proceed to consider a portion of the evidence that has led us to refer the Marly Clay formation of this and other portions of the Plains to the Cretaceous Period.

In the first place we would premise, that with the exception of the gypsum, which is confined to the superior part of the mass, we have not been able to detect any permanent characters that will warrant us in separating the Marly Clay formation into distinct groups. Nearly all the main features mentioned by Mr. Marcon as characterizing the divisions proposed by