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section of Travis County. Even these sections thus exposed, owing to their proximity to the western border, do not represent the Glen Rose beds in their greatest thickness.

Eastward down the valleys of the streams incised below the level of the Lampasas Plain, which but seldom cut entirely through the Glen Rose, the streams become superimposed upon successively higher and higher beds. The lower beds become successively embedded as one proceeds down the streams, and the sections exposed in that direction are constantly being curtailed of their lower beds. Thus sections of the beds in regions of their greatest thickness to the east are only partial, as in the case of the Mount Bonnell section and the Rocky Creek section in Williamson County, where the lower part of the formation is concealed. Finally the streams, in their coastward journeys, completely surmount the Glen Rose beds, which become entirely embedded beneath the Black Prairie, so that beneath the latter area their thickness can be estimated only from the artesian-well drillings, which show them to be 600 feet at San Marcos, 600 feet at Austin, 400 feet at Belton, and 554 feet at Waco. (See chapter on Structure and various geological sections.)

The Glen Rose beds contain fossil remains of foraminifers, echinoids, mollusks, vertebrates, and plants. Corals, brachiopods, and ammonites are conspicuously lacking. The mollusks are the most abundant, but are rarely well preserved, usually occurring as casts and molds. Many layers are barren; others are made up almost entirely of organic remains. In general, the fossils occur in certain conspicuous fossiliferous zones, which have wide extent and are even more persistent than the lithologic matrix in which they are embedded.

In the typical Comanche Peak section the lower 30 feet is marked by numerous mollusks, forming a massive agglomerate, above which is a single stratum, from which one exceedingly rare fish, the only specimen of its genus hitherto found in America (see Pl. XXIV), Macrepstius arenatus Cope, has been procured. A list of the invertebrates of this horizon is given on page 161.

For an interval of from 80 to 100 feet above this lowest fossiliferous horizon the Glen Rose section is comparatively barren, but has not been carefully searched for fossil remains elsewhere. About 100 feet above the base of the section, 250 feet below the base of the Fredericksburg division, and 150 feet below the base of the Paluxy sands of the Comanche Peak section, are thick, white, chalky limestones aggregating about 36 feet, which are exposed in the banks of the Brazos east, north, and south of Granbury. The lower 20 feet of this limestone is a massive agglomerate of fossil requienias embedded in a firm semicrystalline matrix of white limestone. This requienia