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of the beds, especially of the limestones and clays, are the numerous fossil shells which weather out of them and lie strewn upon the surface. (See Pl. XVII.) Some of the shell beds are very conspicuous, making almost the entire material of the rock, such as those composing the oyster hill at Weatherford. Oysters of many species-coiled spirals of the ammonite family, many echinoids or fossil forms of the star-fish family, and numerous other forms-can nearly everywhere be found, except in the sandy beds, and in some places even in these. (See Pls. XXI, XXIV, XXV, XXV I, XXXIII, XXXIV, X.XXV, XL, XLIV, XLVII, XLVIII.) The careful student, and especially the trained paleontologist, will find these fossils invaluable guides in determining the position of the formations in the general series, for they consist of many kinds or species, each of which is limited to some one particular formation or to a limited range in a series of strata.

Upon further consideration of these various beds or strata they are found to be divisible into two great groups or series (see Pl. XVI, p. 110), each of which in turn is composed of many beds of rock. The lower of these series begins with the Trinity sands and ends with the Denison beds, and has been named the Comanche series, after the town of Comanche, where the writer, when a boy, first studied these formations. The upper series begins with the Woodbine formation and extends through the Navarro beds; this has been termed the Gulf series. Each of the series represents a complete cycle of sedimentation and is initiated by an arenaceous littoral terrane-the Comanche series by the Trinity sands, the Gulf series by the Woodbine sands. Each cycle of sediments is the result of continuous (unbroken) deposition during a time when the sea occupied the region.

Logically each series is one continuous formation in the broader sense of that word, which means the continuous product of one uninterrupted geologic event. The individual beds are the stratigraphic units which collectively compose these formations. Usage, however, has decreed that the term " formation" shall be employed otherwise than in its logical sense, and hence the term "formation" will be employed in this paper to designate a conspicuous subordinate stratum, or group of strata, of sufficient individuality to render it a mappable unit.

The beds and series are the only natural units and groups of strata. For convenience it is frequently necessary to discuss as a formation a "

See Am. Jour. Sci., 3d series, Vol. XXXI, April, 1886.