370 Campbell — Rogers's Geology of the Virginias. 



tile soils. At many points, as may be seen between Charles- 

 town and Luray on the Shenandoah Valley Railway, the inter- 

 bedded shales become very much thickened at the expense of 

 the limestones, but the general lithological peculiarities are well 

 preserved at nearly all exposures of the beds along the Great 

 Valley. Many of the limestones of this division are hydraulic, 

 as at Shepherdstown on the Potomac, and on North River, six 

 miles south of Lexington, at both of which points cement has 

 been manufactured. 



The transition from 3b to the upper division, (Chazy 3c), is 

 not very sharply denned, yet we feel assured, in passing from 

 the outcroppings of the lower beds to those of the higher, that 

 we have come upon strata deposited under a decided change of 

 circumstances. The beds consist of a larger percentage of car- 

 bonate of lime, but are less regularly stratified, and have fewer 

 interstratified shales ; a comparatively thin bed of brown sand- 

 stone full of impressions of fossil shells imperfectly preserved, 

 appears among the lower beds, and is very continuous ; then 

 higher up and not far below the base of the Trenton limestone 

 beds of chert of variable thickness are found — sometimes a 

 single bed but often two or three. "In some of these slaty 

 bands, and in the cherty beds so largely interstratified with the 

 limestones of the valley, Gomatites, Ammonites and other re- 

 mains are by no means infrequent, and when found are gener- 

 ally in a beautiful state of preservation." (p. 172). 



The Trenton Group — No. Ill of Rogers, (4a, b, c). — 

 [ Trenton Limestone was included among the " Valley Lime- 

 stones " in the State Reports, but on p. 717 it is put in its true 

 place]. Both the upper and the lower horizons of this group 

 seem to be well defined throughout its whole extent in Virginia. 

 The lower division, Trenton limestone (4a), in many places 

 abounds in corals (ChaBtetes, Columnaria, etc.), while all the 

 classes of Mollusca (Brachiopods, Lamellibranchs, Gasteropods 

 and Cephalopods), have numerous representatives — often in 

 great abundance. In the counties of Augusta, Rockbridge and 

 Botetourt, the lowest beds of Trenton are evidently composed 

 of the debris of an ancient coral reef with some admixture of 

 mollusks and crinoids all cemented by infiltration of carbonate 

 of lime into a solid limestone from 100 to 200 feet thick, of a 

 fine gray color and highly valued for architectural purposes. 



Between the Trenton limestones and the Utica shales (46), 

 there is no well defined plane of separation. The transition is 

 manifested by a rapid diminution in the number and thickness 

 of the fossiliferous limestones, and a corresponding increase of 

 shales, until shales alone constitute the upper portion of the 

 formation. The transition to the highest division, 4c, in the 

 middle counties of the State is marked by changes in both. 



