ml.] VIRGINIA. 815 
hi, but differ in that extensive Layers of chert are interbedded with the 
mal dolomites. The areas occupied by this division may usually be 
[cognized by their topographic features, for the cherts give rise to 
►nspicuous hills or ridges. Chestnut Ridge, Sugar Loaf, and Bet- 
s' Bell are examples of this topography in the vicinity of Staunton, 
it similar ridges and knolls are encountered throughout the valley, 
he Beekmantown age of this series has been determined from gas- 
ropod and cephalopod remains found at various points in the val- 
y, particularly in the vicinity of Lexington, Va. On account of this 
L-aduai passage of the Cambrian into the Beekmantown, the determi- 
ition of the thickness of the latter division is difficult. However, 
le characteristic fossils have been found 300 to 400 feet below the 
>p of the cherty layers, so that their thickness is not less than the 
g^ures mentioned. The difficulty of separating these two divisions 
ithout evidence furnished by fossils is in accordance with a fact 
lat has often been noted, namely, that wherever both are made up 
£ limestone sedimentation has apparently continued through Upper 
ambrian and Beekmantown times without interruption. 
Usually no important pure limestone layers have been noticed in 
lis division, and for this reason, as well as for the reason that the 
>pography often accompanying its exposures is unfavorable to quar- 
ying, the rocks of this age are of little value as a source of Portland- 
3ment rock. In a few instances, however, lenses of comparatively 
ure limestone have been found in this formation as well as in the 
nderlying Cambrian. The following analysis of a sample of this rock 
rom the vicinity of Staunton is typical: 
Analysis of pure limestone from vicinity of Staunton, Va. 
[Charles Catlett, analyst.] 
ilica (Si0 2 ) 1. 79 
lumina (A1 2 3 ) 
ran oxide (Fe 2 3 ) 
ime (CaO) . 50. 36 
lagnesia (MgO) 1. 79 
arbon dioxide (C0 2 ) 41. 36 
Jkalis, etc 3. 97 
TRENTON LIMESTONES. 
Under this general name two distinct series of limestones are here 
ecognized, the older being a coarsely crystalline highly fossiliferous 
ock and the younger the well-known black argillaceous limestones or 
ement rock. The hrst series is well developed in the area south of 
>taunton, where it varies from 60 to 100 feet in thickness, while north 
f Staunton it is apparently missing altogether, for here only the 
ement rocks occupy the interval between the Beekmantown lime 
tones and Martinsburg shales. At first sight these two would appear 
o be but phases of one and the same formation, but this idea is dis- 
