PHYSICAL AND FAUNAL EVOLUTION 75 



the representative of the upper part of Tuscarora, though it may have 

 had a more local origin. 



All of these beds, including the basal white Bald Eagle formation, 

 belong to the much- washed and reworked type of continental sedi- 

 ments, in which concentration of the indestructible quartz had 

 been brought about by long exposure, resulting in the decomposition 

 of all the other minerals, and the removal of the resultant clay and 

 dust by wind and running water. 



The Clinton shales succeed the Oneida conglomerate in Oneida 

 and Herkimer counties. New York, and the Upper Medina quartzite 

 in Avestern New York. In the southern Appalachians, the series is 

 largely composed of sandstones (Rockwood) , highly impregnated with 

 iron, and often containing beds of workable iron. It is generally 

 succeeded by late Siluric (Monroan) or by Helderbergian or later 

 beds, there being a pronounced disconformity at the summit of the 

 Rockwood throughout. That part of the series in Virginia is of 

 continental origin is indicated by the general character of the rocks, 

 but marine intercalations are not uncommon. In some cases in 

 eastern Tennessee the iron ore itself is fossiliferous, having replaced 

 a marine limestone. In such cases the bulk of the formation is shale. 

 In no case is the original thickness preserved since the formation is 

 everywhere bounded above by an erosion plane. In northern Virginia 

 today the thickness is 750 feet (Piedmont folio), and not over 400 feet 

 in southern Virginia. In southern Tennessee and northern Georgia 

 it is from 1,100 to 1,600 feet thick, decreasing westward and north- 

 ward. With our present knowledge of the formations, it is safe 

 to say that the eastern sandy phase represents near-shore deposits, 

 if not actually continental conditions, formed probably at the em- 

 bouchures of several Appalachian rivers; and that westward these 

 deltas merged gradually into true marine deposits, mainly sands and 

 clays, with some limestones intercalated. That the Rockwood repre- 

 sents more than the Clinton of New York cannot be questioned. 

 Where the series is developed in its totality, it probably represents 

 the entire Niagaran, if not a part of the Salinan as well. Along the 

 Alleghany front, fossiliferous shales and iron ores represent this series, 

 with a thickness of not less than 1,000 feet, on the western branch of the 

 Susquehanna. The lower series, 700 feet thick, consists mainly of 



