232 



SCIENCE. 



[Vol. V., No. 111. 



One of the explanations offered for this wave- 

 structure is "that the floor of the Cincinnati 

 sea was acted on from time to time by waves, 

 or similar movements of the ocean-waters;" 

 but it seems just as probable that these ridges 

 were made by the action of waves on the shore. 

 The stratum is made up of fragments of crinoid 

 stems, brachiopods, 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 period of time that a fine sand was formed 

 of crinoidal and shelly fragments. It is well 

 known that the continual clashing of waves on 

 a shore will soon reduce a mass of shells to 

 powder. Dr. Leidy mentions 1 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 collected shells on sea- 

 beaches is aware of the difficulty of procuring 

 perfect specimens after they have been exposed 

 to atmospheric 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 deca}\ For this to be effected, it is 

 necessary that they be covered almost imme- 

 diately, and so deep that they are protected 

 from the air or atmospheric changes. Both 

 these conditions — the one necessary for the 

 preservation of tracks on mud, and the other to 

 entomb 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, perfect 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 organisms, no complete fossils are pre- 

 served. In the stratum above or below they may 

 and do occur. But, while no perfect specimens 

 are found, fragments innumerable remain. 

 Small pieces of crinoid stems, fragments of 

 trilobites and brachiopods, jumbled together in 

 inextricable confusion, are the only signs of 

 fossil organisms. Sometimes the}' lie in heaps, 

 as if thrown together by a swirl in the tide ; 

 sometimes there is only a fragment here and 

 there, and even 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 deposit. 

 These cracks, filled up b}^ a subsequent deposit 



1 Proc. Philad. acad., 1884, p. 12. 



of mud, remain to tell of their origin. These 

 fossil mud-cracks are found in the same localit}*- 

 as the burrows, trails, and other surface-mark- 

 ings. 



Professor Newberry, in the ; Geology of 

 Ohio,' 1 says that the fact that the Cincinnati 

 arch was upheaved before the deposition of the 

 upper Silurian rocks is shown by the strata of 

 the upper Silurian terminating in a feather-edge 

 on each side of the arch, and by the Devonian 

 being so reduced as to render it doubtful if it 

 ever covered the top of the rocks of the Cin- 

 cinnati group. Therefore it is probable that 

 the Cincinnati arch, "during the upper Silurian, 

 and through most if not all of the Devonian 

 ages, . . . formed an island raised above the 

 surface of the sea." 



If this was so, a shore-line would mark the 

 conjunction of the lower Silurian and the Clin- 

 ton ; and along this shore-line would be the 

 place to expect to find such markings as would 

 be made on an ocean-beach. In this regard, 

 Professor Newberry says, 2 — 



"In Adams county the interesting discovery was 

 made by Professor Orton, that a part of the Clinton 

 is formed of a conglomerate of well-rounded lime- 

 stone pebbles and worn fossils of the blue limestone 

 [Cincinnati group] series." 



And lately Mr. U. P. James has found a 

 slab of rock near the top of the rocks of 

 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 of 

 New York is the one where most of the trails, 

 burrows, and beach-markings have been found. 

 Professor Hall says in regard to them, 3 that 



"They occur in greater or less number and per- 

 fection throughout the entire extent of the group;" 

 and that (Ibid., p. 26) "from the character of the 

 surfaces of the arenaceous beds in which they occur, 

 I am inclined to the belief that many of them were 

 made while the bed was exposed above water, and 

 most of the others in very shallow water. In many 

 instances the marks of what appear to be wave-lines 

 are still preserved upon the surface of the layers. 

 These markings have been regarded as a line of beach 

 at the period of the Medina sandstone ; and the strata 

 under consideration follow in immediate succession 

 to that period. They are, moreover, associated with 

 pebbly beds which were probably littoral." 



Thus, if the markings and the fossil remains 

 found in the Clinton are to be regarded as made 

 on exposed surfaces, and if these same mark- 

 ings, or similar ones, are found in the rocks of 



* Vol. i. pp. 94 et seq. 2 Ibid., p. 103. 



3 Paleontology of New York, vol. ii. p. 27. 



