H. D. Campbell — Cambro-Ordovician Limestones. 445 



Art. XL-Y. — The Cambro-Ordovician Limestones of the Mid- 

 dle Portion of the Valley of Virginia ; by H. JD. Campbell. 



Neither the Knox dolomite nor the Shenandoah lime- 

 stone, if used as the name of a geologic formation, should be 

 made to include all of the Cambrian and Ordovician limestones 

 of the Y alley of Virginia from Tennessee to Maryland. 



M. R. Campbell* makes the Shenandoah limestone of south- 

 west Virginia comprise not only the Knox dolomite but at least 

 1500 feet of Cambrian strata beneath it. He also describes 

 two formations of limestone above the Shenandoah and recog- 

 nizes 500 feet of subjacent variegated shale and impure lime- 

 stone. 



At the border between Tennessee and Virginiaf he makes 

 the Shenandoah limestone include not only the Knox dolomite 

 but five other formations, remarking that the six merge into 

 one formation which prevails along the eastern side of the 

 Appalachian valley at least as far as Pennsylvania. 



It is not the purpose of this article to discuss the correlation 

 of the Knox dolomite and the Shenandoah limestone, which 

 can be satisfactorily accomplished only after several additional 

 sections across the valley of Virginia have been described in 

 detail and the fossils from fixed horizons have been com- 

 pared. 



This introduction is offered as an explanation for using the 

 following entirely new names for the formations recognizable 

 in the limestones of the portion of the Appalachian valley 

 near Lexington and the Natural Bridge. Virginia. 



Section of the Valley Limestones near Lexington, Virginia. 

 Ordovician -j 



Period. Name of formation. Thickness in feet.' 



Liberty Hall limestone , 1000 ± 

 Murat "limestone 100-150 



Natural Bridge limestone 3500 -f- 

 Buena Vista shale 600-900 



Sherwood limestone 1600-1800 



Cambrian 



! 



Sherwood limestone. — In the bluff of James River at Sher- 

 wood, Va. and for more than twelve miles to the southwest, the 

 lower part of this formation consists of several hundred feet 

 of white crystalline dolomite. This dolomite is overlaid by 

 heavy beds of light blue and gray magnesian limestone with 

 occasional beds of shale and shaly limestone. It was just 

 beneath or at the very base of the Sherwood limestone that 



*U. S. Geol. Surv., Geol. Atlas of U. S., Folio No. 26, 1896. 

 flbid., Folio No. 59. 

 Am. Jour. Sci.— Fourth Series, Vol. XX, No. 120. — December, 1905. 

 31 



