1854.] 7 



The hills which separate these Two ancient alluvial valleys are a continuous 

 chain, and of sufficient altitude to constitute an effectual barrier to the mingling 

 of the waters of these two rivers, at any point nearer than their present con- 

 fluence at the city of Pittsburgh- I observed at Rochester, New Brighton, and 

 at different points along the Monongahela and Allegheny rivers, that this deposite 

 reposes on a dark colored shale, sometimes passing into more compact slate. As 

 I was descending a hill on the Frankstovvn road, near East Liberty, I discovered 

 an old acquaintance, a thin stratum of calcareous rock, on which the shale re- 

 poses, and which in contradistinction to other limestones, belonging to the coal 

 series I have denominated the ^rj^a-limestone. 



This is a somewhat peculiar rock, characterised by containing a vast number 

 of small bivalve shells, belonging to the genus Atrypa, etc. When long exposed 

 to the atmosphere, its external surface readily disintegrates, and presents a rag- 

 ged and shaly appearance, the surface being covered with small Atrypa and pro- 

 jecting joints of Encrinites. 



In some localities I have found in this rock the genera Terebratula, Trilobites, 

 Plei/.rotomaria, Productus, Spirifer, Bellerophon, Euomphalus, Ammonite and 

 Orthoceratite. But the joints of Encrinites and the small Atrypa are the charac- 

 teristic fossils, and distinguish it from all other limestones belonging to the car- 

 boniferous series. This limestone, in Westmoreland county, is seldom found 

 more than two or three feet in thickness. But near the Ohio line I found it over 

 ten feet, it being there a very compact and durable rock, well adapted for agri- 

 cultural and other purposes. It is exposed at numerous points in Western Penn- 

 sylvania, and uniformly presents similar lithological and paleontological cha- 

 racters. 



I mentioned, at the commencement of this paper, that this ancient alluvium 

 was traceable everywhere along the Ohio river and its tributaries which I have 

 examined, and doubtless along those which I have not so closely surveyed. Those 

 tributaries to which my attention has been more particularly directed, are the 

 Beaver, Allegheny, Monongahela, Youghiogeny, Kiskeminitas, Loyalhanna and 

 Conemaugh rivers, etc., etc. But in the brief and necessarily imperfect delinea- 

 tion of these remarkable deposites, which I have attempted to give, I have con- 

 fhied myself to a few localities only, leaving a more extended and minute 

 description for some future period. 



It is not at all improbable that those high bluffs which bound the vast alluvial 

 valley of the Mississippi river (which is from thirty to fifty miles in width), 

 were deposited at the same period. Also the alluvial terraces on which the city 

 of Cincinnati is built, the highest of which is 120 feet above the level of the 

 Ohio river; and doubtless the natural terraces and ridizes of Lake Erie and 

 Ontario, which were so graphically delineated by Mr. Charles Whittlesey, of 

 Cleveland, in the July No., 1850, of the American Journal of Science. 



For, in all these localities, fossil teeth and other bones of the mammoth and 

 mastodon have been found imbedded in this deposite. These animals, which 

 were once the lords of the creation, the primates of this lower world, perished 

 and became extinct in that last grand revolution, during which I presume these 

 ancient sedimentary deposites were slowly and gradually elevated to their pre- 

 sent position, and which immediately preceded the advent of man. 



From the whole of this investigation my mind has been convinced that 

 the relative position of the rivers and this ancient alluvium through which 

 they now pass, or along whose borders their waters deeply flow, has changed 

 since the period of deposition. But when, and in what manner, or by what 

 cause, are questions not so readily answered as proposed. 



The hypothesis of local oscillations, first proposed many years ago by Sir 

 Charles Lyell, to account for the deposition of the Rhenish loess, will, with 

 slight modifications, I believe, clearly explain all the phenomena connected with 

 this post tertiary formation. 



Indeed, from the numerous facts upon which this hypothesis is based, it may 

 be regarded as an exceedingly plausible theory, if not a legitimate induction. 



It has lonj; been ascertained that various parts of the earth's surface are now, 

 and perhaps ever have been, undergoing slow and gradual oscillatory movements. 



