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twenty to forty per cent. The Rocky Comfort beds are also the only ones of considerable extent which have these peculiarities, those at White Cliff being the only others known to bear even a partial resemblance to them. The western border of this chalk commences a few miles west of the southwest corner of the State of Arkansas, in Indian Territory, crossing Red River (the exposures continuing up the south side of the valley of that stream to the north of Sherman, where it deflects southward), passing near Whitesboro, Sherman, McKinney, Dallas, Hillsboro, Waco, Belton, Austin, San Antonio, and Spofford's Junction, Texas, beyond which it bends northward, appearing in the disturbed mountains in the vicinity of El Paso and the New Mexican realm, and again in No Man's Land, Kansas, Nebraska, and Colorado, where it is closely related and probably identical with the Niobrara chalk of Meek and Hayden.

"A great portion of the former extent of this chalk has been destroyed by erosion, and its western border in Central Texas is now receding eastward under the influence of excessive atmospheric decomposition and denudation. From Austin to San Antonio it is more stable, but west of the latter place erosion again becomes great. That the whole group once continued far to the west, and perhaps entirely across the State, is not at all improbable.

"The characteristic topographic and physical features of this formation as seen at Rocky Comfort, such as the gently undulating topography, the white crumbling exposures, the intense blackness of the soil, are so nearly identical with those of the same formation in Texas that they are indistinguishable."

In the vicinity of Austin the soft and chalky structure is somewhat destroyed by the volcanic disturbances of the vicinity, such as the co-deposition of volcanic ash, and excessive jointing and faulting, but it maintains its pure chalky aspect elsewhere.

The Austin-Dallas chalk is succeeded by a remarkable deposit of clays, aggregating some twelve hundred feet in thickness, according to reported well borings and estimates of the normal dip. These clays occupy the whole of the main Black Prairie region east of the Austin-Dallas chalk, and form the basis of the rich black waxy soil. Notwithstanding their areal extent, good outcrops of the unaltered structure are seldom seen, owing to the quick decomposition into soil. However, at the Blue Bluffs of the Colorado, six miles east of Austin, a superb exposure is afforded, where these clays can be readily studied and diagnosed.

"The Neozoic Geology of Southwest Arkansas." Vol. 2 of the Annual Report of the State Geologist of Arkansas. Little Rock, 1888, pp. 90-95.