Page:AmJourSci 4 38 228 507-510.djvu/1

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Barbour—Carboniferous Eurypterids of Nebraska.

Art. XLV.—Carboniferous Eurypterids of Nebraska; by Erwin H. Barbour.

During the field season of 1912 the Nebraska Geological Survey found a bed of eurypterids about one mile south of Peru. This adds one more to the list of known eurypterid localities, and adds one form believed to be new. Nowhere apparently are eurypterids found in such variety and in such excellent preservation as in New York State, yet the Nebraska beds bid fair to make an unusual showing numerically.

The eurypterid shales of Nebraska, as far as known, outcrop one mile south of Peru, in the Coal Measure bluffs facing the Missouri River. The alternating limestone and shale in the bluffs at the edge of the town change rapidly to shale, which becomes increasingly arenaceous and merges finally into massive sand some forty to fifty feet thick about a mile south of Peru. Here the eunypterid bed is found. The walls and bluffs are vertical, because only recently they were deflection banks of the Missouri River. The tracks of the Burlington Railroad run close to the base of the bluffs. About ten feet above the tracks, and about thirty feet above the river level, may be noticed in the massive sand a distinct shaly band scarcely a foot thick, and two to three hundred feet long. This local band is composed of thin, irregular, shaly layers, seldom a half inch thick, alternating with micaceous sand. The shale seams in the topmost two inches yield the eurypterids. These thin shale seams cleave readily, exposing the eurypterids and numerous associated plants. Because of shrinkage joints, the shales tend to break into rather small blocks, and unfortunately many good specimens are damaged or destroyed.

Since removing a dangerous overhanging ledge the work of collecting has been rendered quite safe and easy, and about forty specimens were found on as many square feet of shale, or approximately one to each square foot. Their chitinous shells are reduced to mere carbonaceous films sometimes scarcely distinguishable from the shaly matrix. Accordingly, some specimens are faint. Others are distinct, however, and well differentiated from the slate-colored background. Some specimens are so well preserved that the details of the gross anatomy may be made out, and even some of the ornamentation and minute sculpturing.

Associated with the eurypterids at Peru are innumerable leaves, stems, and fragments of certain land plants, conspicuously Neuropteris pinnules, stems of Calamites, and leafwhorls of Asterophyllites. The last mentioned add to the beauty of some of the specimens in our collection, because the Am. Jour. Sci.—Fourth Series, Vol. XXXVIII, No. 228.— December, 1914.