Page:The Habitat of the Eurypterida.djvu/151

Rh everywhere and as Schmidt puts it, "In the extent of a single $$\tfrac{1}{4}$$ verst one may here lay out places for eurypterid quarries to one's heart content" (248, 29).

Above the plattenkalk horizon is a brecciated limestone of no great thickness consisting of angular or slightly rounded fragments of compact limestone in a matrix of similar limestone which contains Calamopora polymorpha. The breccia is not derived from the underlying dolomite, according to Schrenk (254, 47). This physical evidence, of a break at the top of the eurypterid dolomite has been more fully described from other localities, as, for instance, at Wita, the section next to be considered.

To the southwest of Rootziküll is the village of Wita. Here in the yellowish white dolomite which is the characteristic eurypterid-bearing facies two quarries have been opened. It was found that the eurypterids occurred not only in the dolomite, but also at a higher horizon in a brecciated coral limestone which is made up of angular, sometimes rounded white nodular masses which are for the most part corals lying embedded in a uniform, yellow, marly limestone matrix. Schmidt (241, 167, 168) would correlate this bed with the Burgsvick oölite of Gotland, the formation which there marks the break between the upper and lower Gotlandian. The limestone at Wita is only one foot thick; in its upper part it contains Leperditia baltica, Turritella obsoleta (= Holopella obsoleta), Spirifer elevatus, and certain corals, all being characteristic of the Upper Ludlow of England and of the Upper Gotlandian of Gotland. In the lower portion of the breccia occur: Cephalaspis verrucosus, C. schrenkii, Eurypterus fischeri, Bunodes lunula, a new crustacean Dithyrocaris ? sp., Orthoceras bullatum ?, Lingula nana, and Palæophycus acicula, besides many fragments of crustacean claws, segments of walking legs and the like. The section is of importance for three reasons: (1) There is physical evidence of a break at the end of Wenlock or Lower Ludlow time, marking a retreat of the sea. It did not return until Upper Ludlow time as indicated by the presence of fossils of that age in the matrix of the brecciated limestone. (2) The eurypterids occur abundantly in the beds deposited immediately after the normal marine conditions ended, while the sea retreated, and at the time when dry land was being enlarged and consequently rivers were extending their distal portions. (3) The eurypterids are also sparingly found in the breccia and conglomerate which marks the return of the sea and renewed deposition of marine sediments with marine organic remains. (4)