110 INDEX TO THE STKATIGRAPHY OF NORTH AMERICA. 



The Tomstown limestone * * * - is composed largely of limestone, both massive and 

 thin bedded, in part cherty, with some shale inter bedded near the base. * * * [The more 

 characteristic fossils which it has yielded are Salterella sp. undet., Kutorgina n. sp., and frag- 

 ments of Olenellus. These definitely determine its age as Georgian (Lower Cambrian). — 

 G.W.S.] 



In central Virginia a formation 1,600 to 1,800 feet thick, occupying about the same interval 

 but apparently including beds which in the Chambersburg area are calcareous sandstones and 

 are mapped with the overlying Waynesboro formation, has been named Sherwood limestone by 

 H. D. Campbell. * * * 



The Waynesboro formation is a series of sandstones, purple shales, and limestones overlying 

 the Tomstown formation. At the base are very siliceous gray limestones that weather to 

 slabby porous sandstone, large round masses of rugose chert, and white vein quartz. In the 

 middle of the formation are dark-blue to white subcrystaUine limestones and dolomites^ which 

 become sUiceous upward and merge into mottled slabby sandstone and dark-purple siliceous 3hale 

 at the top. * * * j^ fe^ poorly preserved shells, two of which are identified as Obelus (Lingu- 

 leUa) sp. undet., obtained from sandy shale at the very top of the formation, suggest Acadian 

 (Middle Cambrian) age but are not conclusive. 



In central Virginia the Buena Vista shale, described by H. D. Campbell, is at this general 

 horizon, but, as previously stated, apparently has a different lower limit. It is described as 

 bright variegated shale, 600 to 900 feet thick, with mottled limestone and shale in the lower 

 part. Mr. Walcott found in it a species of Ptychoparia related to Acadian (Middle Cambrian) 

 species of Tennessee. 



The Elbrook formation is the thick series of gray to light-blue shaly limestone and calcareous 

 shale that overlies the purple Waynesboro formation. The formation is decidedly shaly, most 

 of the included limestones being minutely laminated and weather readily into calcareous shaly 

 plates. * * * Near the middle are massive beds of dolomite and very siliceous or quartzitic 

 limestone that weathers to porous slabby sandstone and frequently forms knobs and ridges. 

 The formation is limited above by limestone conglomerates containing rounded vitreous quartz 

 grains and others containing tabular fragments of limestone, which characterize the base of the 

 overlying formation. * * * The only fossUs found in this formation were fragments of 

 trUobites, which suggest Acadian. 



The Conococheague limestone is characterized by beds containing thin sandy laminse 

 and quartz grains that weather into hard shale fragments and thin slabby sandstones which 

 generally give rise to rocky hills and rugged topography. 



The base of the formation is usually easily determined because it is marked by siliceous 

 beds and conglomerates that produce a ridge. The conglomerates are of two kinds; one is 

 composed of rounded limestone pebbles, 1 inch or more in size, in a matrix containing numerous 

 round coarse grains of vitreous quartz; the other is composed of long slender fragments of 

 limestone in a calcareous matrix, which, because the fragments are tilted at various angles, 

 is called "edgewise" bed. Interbedded with the conglomerates are oolites and dark shaly 

 limestones with red clay partings. 



The body of the formation is a closely banded dark blue limestone, the bands varying from 

 one-half inch in width to minute laminse. The banding is inconspicuous in the fresh rock 

 but is brought out in weathering as yellowish sandy streaks across a light-blue or gray sm-face. 

 Toward the top these partings become more numerous and sandy, and weather into hard 

 sandy plates and sheets. Chert is not an important constituent of the formation in the Cham- 

 bersburg and Mercersburg quadrangles. * * * ^he fossUs found in this formation com- 

 prise DiJcelocephalus hartii Walcott; D. sp. undet.; and Billingsella like B. desmopleura. The 

 trilobites place this part of the formation definitely in the Saratogan (Upper Cambrian). In 

 the basal conglomeratic beds a species of Cryptozoon, probably O. proliferum Hall, characterized 

 by a mammiferous surface, the elevations one-half to 1 inch in diameter, is rather generally 

 present. 



