e 
314 cement materials and industry. [bull. 243. 
argillaceous limestone, the Trenton cement rock. Members 1 and % 
are apparently uniformly developed throughout the valley, but 3 and 
4. although widely distributed, are sometimes absent. 
CAMBRIAN LIMESTONES. 
( )n account of the lack of continuous exposures and the difficulties 
in distinguishing the various beds the thickness of this division lias 
not been definitely ascertained, but it is certainly not less than 1,000 
and may exceed 2,000 feet. Fossils are practically absent in these 
rocks in this part of the valley, but farther north, notably at several 
localities in Pennsylvania and New Jersey, a sufficient number o 
determinative fossils have been found to indicate that probably th 
entire division is of Upper Cambrian age. These limestones are 
underlain by a quartzite containing Lower Cambrian fossils, so that 
although the two formations are apparently conformable there is 
great time break between them. 
These Cambrian limestones are massive bedded, vary from dai 
gray to light gray or light blue in color, and are nearty always highly 
magnesian in composition. Toward the base purple or silvery shales j 
are sometimes seen, but as a rule the entire formation is one of heavily j 
bedded magnesian limestones. 
On account of the high percentage of magnesia these limestones are 
of no value for the. manufacture of Portland cement, but their compo- 
sition does not preclude their use in making natural cement. At the e'j 
plant at Glasgow, in Rockbridge County, natural cement has been i ; 
burned for many years from the magnesian limestones of the lower rj 
part of this division. Cement from this plant was used in building- 
the locks of the James River and Kanawha Canal. 
Occasionally, however, strata of pure nonmagnesian limestone are 
interbedded with the more t}^pical magnesian rock, and it is these j 
strata that will prove valuable in Portland-cement manufacture. Such 
strata have been observed in various parts of the valley, but their rj 
outcrops are more or less scattered. On account of this fact and of If 
the geologic structure of the entire formation and the small per cent i 
the pure limestones contained in it, these nonmagnesian strata can not I 
be definitely mapped and must be determined in the field. In New j 
Jersey and Pennsylvania there is the same arrangement of a few strata 1 
of nonmagnesian limestone with a great series of dolomite, and the el 
former is the source of part of the limestone used in the Lehigh district 
to bring to the cement rock the required percentage of calcium car- 
bonate. Near Annville, Pa., these nonmagnesian limestones occur 1 
in greater quantities than usual, and here much of this limestone is 
quarried for shipment to cement plants. 
BEEKMANTOWN LIMESTONE. 
The Cambrian dolomitic limestones grade upward imperceptibly into 
another series of strata that have essentially the same chemical composi- 
