238 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



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 disconfonnity at the summit of 

 the Rockwood throughout. That part of the series in Virginia is of continental origin is indi- 

 cated by the general character of the rocks, but marine intercalations are not uncommon. In 

 some cases in eastern Tennessee the iron ore itself is fossihferous, having replaced a marine lime- 

 stone. 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 to-day the thickness is 750 feet (Piedmont foho), 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 northward. 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 embouchures of several Appalachian rivers; and that west- 

 ward these deltas merged gradually into true marine deposits, mainly sands and clays, with 

 some limestones intercalated. That the Rockwood represents more than the Clinton of New 

 York can not be questioned. Where the series is developed in its totahty, it probably repre- 

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

 fossihferous 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 fissile shales, including an iron sandstone, and 'with Buthotrephis in the upper part. This is 

 succeeded by 110 feet of calcareous fossihferous shales; and this by 230 feet of fossihferous 

 shales and limestones with a Niagaran fauna. Above this foUows 350 feet of red shales, prob- 

 ably representing the Upper Salina and separated by a hiatus from the fossihferous Niagaran 

 shales. 



In eastern New York, at Swift's Creek, the type locahty for the Clinton, this formation is 

 226 feet thick and is followed by 5 feet of Niagaran and then by the red shales of the Upper 

 Sahna. On the Niagara River the Clinton shale with the two succeeding limestones has a total 

 thickness of 32 feet, followed by 68 feet of Rochester shale. The total of the Niagaran, includ- 

 ing the Guelph, is from 270 to 325 feet, as shown by borings. This is foUowed by Lower Saliaan. 

 In the Rochester region the Clinton has a thickness of 80 feet, including the Irondequoit or upper 

 limestone (17 feet), which Chadwick refers to the Rochester. The eastward thinning of the 

 Upper Niagaran beds indicates either that these beds were eroded before the deposition of the 

 red shales, probably during the Shawangunk epoch, or that the Rochester-Lockport of the west 

 is in part represented by Upper Clinton in the east. The Guelph element may never have 

 extended to the Chnton type region, which may have been above water and so subject to 

 erosion. 



Local details are given in papers by Darton,^^^"^^^ Ries,*" Clarke,"* Grabau,"*-^^" 

 Hartnagel,*"^"^^^ and Merrill ^^^ published in the New York State reports and 

 elsewhere. 



A critical and comprehensive report on the Silurian formations in New Jersey 

 has been made by Weller.*^"'' The following is a condensed section of the forma- 

 tions which he distinguishes from the base up. 



Shawangunk conglomerate: Coarse quartz conglomerate, of white or yellowish pebbles Feet. 



in a gray or red matrix; unfossiliferous; unconformable on Hudson (Ordovician) "shale. 1,500-1,600 

 Medina-Longwood sandstone: Soft, often somewhat shaly, and deep red; unfossiliferous. 2,300-2,400 



Poxino Island shale : Greenish, calcareous, unfossiliferous 1-200 (?) 



Bossardville limestone: Fine-grained, compact bluish-gray banded limestone, unfos- 

 siliferous 12-100 



Decker Ferry formation: Sandstone with thin calcareous beds or limestone with some 

 thin bands of greenish shale; highly fossiliferous; divided into three faunal zones and 

 the uppermost into three subzones; correlated with the "Coralline" (Niagaran) lime- 

 stone of eastern New York; presumably contemporaneous with but faunally not closely 

 related to the Niagara limestone of the Mississippi Valley 50 



