316* CEMENT MATERIALS AND INDUSTRY. [bull. 243 
gton L 
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i thi i , 
proved by the development of both series in the vicinity of Lexin 
Va. Fossils are abundant throughout both series, the first often be 
crowded with ramose bryozoa and masses of Solenopora, while brachio j 
pods, ostracods, and trilobites of Trenton age predominate in tin 
second. Although samples of the coarsely crystalline limestones rii 
high in lime, the strata as a whole contain so much chert that they aa^ 
of little value for mixture with the cement rock. 
Character of cement rock in valley. — The youngest member of th 
Shenandoah limestone, the argillaceous Trenton, or " cement rocklj 
usually resembles the corresponding strata in the Lehigh district mom 
in chemical composition than in physical aspect. In Pennsylvania thj 
cement rock is usually a dark-gray or black, slaty limestone, which, d 
account of the shearing to which it has been subjected, breaks undei 
the hammer into flat pieces with smooth, glistening surfaces. As thiil 
rock loses its argillaceous character — i. e., as the percentage of limit 
carbonate in it increases, it loses the slaty appearance and becomes \ 
light-gray crystalline limestone. In the valley, however, the metaJ 
morphism seems not to have been so great and the aspect of the lime f ' 
stones varies according to their composition. For example, the rockJf 1 ' 
of the formation outcropping near Woodstock are little more tharr 
compact hardened strata of calcareous mud while, on the other handl »' 
the same horizon on the western side of the valley is occupied bj>] stl 
tough, crystalline dark blue or black limestones. st 
South of Staunton, especially in the vicinity of Lexington, thn'r 
Trenton strata have been closely folded and compressed and show ffl 
considerable metamorphism. The result is that the argillaceous lime* 1 " 
stones in that locality resemble those of the Lehigh district more thanlP 1 
at any other point in the valley. 
Analyses of these rocks are given under the discussion of th«n K 
localities in detail. 
General distribution of argillaceous Trenton limestone. — On accoun 
of the geologic structure of the valley, the argillaceous limestone 
are found in three well-defined belts. Two of these belts are forme< 
by the outcropping edges of the syncline that forms Massanuttei 
Mountain, while the third follows the western edge of the valley 
Exposures of the easternmost belt are found at numerous places alon^i 
a northeast-southwest line extending from a point about 5 miles eas- 
of Woodstock to Fishersville. The next belt to the west parallels thiii 
and shows many outcrops along a similar line from Woodstock U 
Staunton. At several places along these lines of outcrop the argilla; 
ceous limestone is missing. The arrangement indicated is most pro- 
nounced (1) in the region just south of Massanutten Mountain, having 
McGaheysville on its eastern edge; and (2) in the area south of Staun 
ton, bounded b}^ Staunton, Barter Brook, and Fishersville. The thin 
belt occurs along the western edge of the valley and parallels the othe 
