Page:Popular Science Monthly Volume 77.djvu/77

Rh the arrival of the Chemung fauna; and in western New York no trace of the Catskill type of sediments appears till after the close of the Devonian.

These facts are direct evidence of shifting of the environmental conditions of the edge of the continent westward as the deposits of the middle and upper Devonian were being laid down. With this shifting westward of the off-shore conditions of the sea, there went on a corresponding shifting of several faunas that were adjusted to each phase of those conditions.

These facts were stated in a paper on the classification of the upper Devonian published in 1885.

§ 2. The Appearance of Dominant Species of a General Fauna in Reversed Order of Succession at the Close of a Fossiliferous Zone.—The case of Spirifer lævis in the Ithaca Zone and of the frequent appearance of Leiorhynchus at the opening and close of a fossiliferous zone were among the earliest observed facts suggesting the actual shifting of the body of the fauna entering the area in one order of succession and its departure in the reverse order. In the Ithaca section there occurs at the base of the fossiliferous zone of the Ithaca member a bed containing abundance of Spirifer (Reticularia) lævis. The discovery of the same species at the top of the fossiliferous zone as the normal Ithaca fauna become sparse gave the first suggestion that the faunas were moving or shifting. The Reticularia zone marked the first trace of the fauna to enter and the last to leave the area. Confirmatory evidence was also found in the order of succession of the dominant species of the Ithaca fauna. These facts were reported in 1883.

§ 3. The study of the mode of occurrence of Leiorhynchus still further drew attention to the definite order in which series of species came in and went out of any given area. The species of the genus were generally found abundantly at the base or at the top of the fossiliferous zones rich in the brachiopods in the midst of which Leiorhynchus was rare.

§ 4. The reappearance in a single or few strata of several representatives of an earlier fauna long after the formation to which they were normal had ceased.

Slight traces of this fact were observed in the first survey of the Devonian section passing through Ithaca, reported in 1883, Bull. 3, U. S. G-. S., and the fauna ~No. 14 N (p. 15) was called a recurrent Hamilton fauna because of the appearance there of such species as Spirifer fimbriatus, S. augustus, Pleurotomaria capillaria and others;