OF THE NEW RED SANDSTONE FORMATION OF PENNSYLVANIA. 
7 
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¬ 
form ably to the primary rocks beneath. To the eastward of this 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 he 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 brecliia , 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 columnsof 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. 
8 
