Page:Science vol. 5.djvu/256

 [Vol, v.. No. MI.

��One of tlie oxplanaliotis ofiercd for this wave- structure is " that the floor of the Cincinnati aea was acted on from time to time by waves, or similar movements of the ocean-waters;" bill it seems just as probable that these ridges were made by tho action of waves on the shore. The stratum is made up of fragments of crinoid stems, brachioiKids, and other forms of life. It is just as if it had been exposed to the ac- tion of the weather and the waves for such a long perIo<l of time that a fine sand was formed of crinoidal and shelly fragments. It is well known that llie continual dashing of waves on a shore will soon reduce a mass of ahella to powder. Dr. Leldy mentions' that while at Atlantic City, the beach, after every storm, is strewn with immense numbers of shells : in a short time these become reduced to fragments, and eventually disappear.

Every one who has collecteil shells on aea- bcaches is aware of the difficulty of procuring perfect specimens after they have been esposed to atmospheiic agencies for a short time. Still in some places, notably in the Bay of Fundy, tracks left on the mud, raindrop impressions, traces of leaves, and other marks, are preserved in a perfect state. At the same time it seems unlikely that organic matters will be preserved from decay. For this to be effected, it is necessary that they l)e covered almost ijnmc- clialcly, and so deep that they are protected from the air or atmospheric changes. Both these conditions — the one necessary for the preservation of tracks ou mud, and the other to enlomb perfect organisms — seldom seem to occur at the same time and in the same place : consequently it rarely happens that in the stratum where surface- marks, burrows, and trails occur, jwr/eci fossils of any sort are found. While the whole surface of immense slabs of rock may be covered with trails, burrows, or impres- sions of ot^anisms, no complete fossUs are pre- served. In the stratum above or lielow they may and do occur. But, while no perfect sjwcimens are found, fVagments innumerable remain. Small pieces of crinoid stems, fragments of trUobitea and brachiopoils, jumbled together in inextricable confusion, are the only signs of fossil organisms. Sometimes they lie in heaps, as if thrown together by a swiil in the tide ; sometimes there is only a fri^ment here and there, and evcu it shows unmistakable signs of the action of the weather.

Mud-cracks, too, evidence the fact that the surface was exposed to the action of the sun long enough to dry and crack the de|>oait. These cracks, filled up by a subsequent deposit

> ITm. Phll^. aiad., 1881, p. 12.

��of mud, remain to udl of their origin. TUest fossil mud-cracks are found in the same locality I OS the burrows, trails, and other surface mark* a

��Newberry, in the Mleology Ohio.'' says that the fact that the (.'incinni arch was upheaved before the deposition of tt upper Silurian rocks is shown by the strata a the upper Silurian terminating in a feather-edge * on each side of the arch, and by the Devonian being ao reduced as to render it doubtful if it ever covered the top of the rocks of the Cin- ciuaati group. Therefore it ta probable that the Cincinnati arch, " during the upper Silurian. and through most if not all of the Devonian ages, . . . formed an island raised above " surface of the sea."

If this was so. a shore-line would mark I conjunction of the lower Sihu'ian and the Clin-'] ton ; and along this shore-line would be t place to expect to find such markings as wouli be made on an ocean-beach. In this regard Professor Newberry saya," —

" in Adanu county the interesting tliscoTery w tiiiule by Froressor Orton, tbat a part of Ibe Clialoi is fomied of a conglomerate of vell-rauuded liOT' stone pebbles unil worn fossiU of the blue limeslo ICiueiniiati group] serieB,"

And lately Mr. U. P. James has found slab of rock near the lop of the rocks the Cincinnati group which shows well-marked and unmistakable impressions of raindrops, marks which could not, by any possibility, have been made and preserved, except on an exposed surface.

It is well known that the Clinton group New York is the one where most of the Irwl burrows, and beach-markings have been foui Professor Hall saya in regard to them,' that

��"They occur in greater or less number and . fectioii througliout the entire extent of the groupj-_ and thnt (Ibid., p. 28) "from tbe character of tat surfaces of ilie nreiiaceom beds in wbich they occur, I am Inclined lo the belief that many of them were made while Uie bed was exposed above wBt«r, and most of the others In very shallow water. In man; instances the niarts otwiiat appear to be loaoe-Ifnei are still preserved upon the surface of the layers. These markings havebeeu regarded as a line of beach at the periml of the Medina sandstone; and the strata under consideration follow in Immediate succe"' — to that period. They are. moreover, associated pebbly beds which were probably littoral."

Thus, if the markings and the fossil rcmatn found in the Clinton are to be i-egarded as n on ex|X»sed surfaces, and if these t ings, or similar ones, are found ii

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