OF THE NEW RED SANDSTONE FORMATION OF PENNSYLVANIA. 189 



mountains, in a slightly interrupted, rather curved line, to the Hudson river at Stony- 

 Point., its greatest breadth being about thirty miles, and always resting uncon- 

 formably to the primary rocks beneath. To the eastward of thi.s it appears in the 

 Valley of the Connecticut, and extends through Massachusetts, north, near to the 

 Vermont State line. 



The first notice of the existence of this red sandstone, seems to be in the 

 Transactions of the American Phil. Soc, in 1799, by Th. P. Smith. In examining 

 the " basaltes" of the Conewago Hills, he found " they were interspersed with large 

 masses of hrechia, composed of silicious pebbles, evidently rounded by friction, 

 imbedded in the red freestone of our mountains." These pebbles were probably 

 calcarious, not silicious, and the same now known as Potomac Marble. Chief Justice 

 Gibson, in a paper, on the Trap Rocks of the Conewago Hills, in the same 

 Transactions, 1820, followed Mr. Maclure's views in considering this the Old Red 

 Sandstone. 



This red sandstone formation was considered by Mr. Maclure, in his Geology of the 

 United States,* to be analogous to the Old Red Sandstone of Europe ; but that error 

 was, a long time since, obvious to the Geologists of this country. The 

 great difficulty which presented itself was in the absence of organic remains, these 

 not having then been observed. The lithological characters are so nearly the same 

 with the Old Red Sandstone, that, relying on them only, the mistake was very 

 natural. 



Subsequently, organic forms were observed in the imprinted foot-marks of Birds 

 and Batrachians, by Dr. Deane and Prof. Hitchcock, — heterocercal fish by Mr. 

 Redfield, and some obscure fucoids by Prof. Mather, as well, also, thin seams of 

 ligniform coal. These were followed by Prof. H. D. Rogers having observed 

 " distinct impressions of Encrini," in the fragments which composed the calcareous 

 conglomerates, used, under the name of Potomac Marble, in the columns of the Senate 

 Chamber at Washington. The origin of these fragments. Prof. W. B. Rogers refers, 

 at their nearest source, to the great Valley west of the Blue Ridge in Virginia, which 

 ridge in Pennsylvania is known under the name of the South West Mountains, 

 or Conewago Hills. This valley is formed of the earlier palaeozoic rocks, and 

 embraces formations No. 1, 2 and 3, of the Pennsylvania State Reports. They are 

 equivalents of the Potsdam Sandstone, Calciferous Group, and Black River Lime- 

 stones of the New York Geologists. These, lying contiguous on the western border 

 of the Red Sandstone formation, would naturally present the materials for such a 

 deposit, and, therefore, we have that which appears to be the result of the forces 

 in action at the time, breaking into fragments, and rolling into forms more or less 

 irregular, the component rocks of the older strata of the district which now forms 

 the valley, the present intervening range of mountains having been subsequently 



* Transactions of the American Philosophical Society, vol. 1, new series. 



48 



