118 ECONOMIC GEOLOGY OF AMITY QUADRANGLE, PA. 
LIMESTONE. 
RELATIVE ABUNDANCE. 
The rocks of the Monongahela and Washington formations present 
a striking contrast to the underlying Carboniferous rocks in the pro- 
portion of limestone they contain. While in the Pottsville, Allegheny, 
and Conemaugh formations beds of limestone are rare and, with a few 
exceptions, very thin, in the Monongahela and Washington deposits 
they are frequently found up to 20 or 30 feet in thickness, and in the 
Monongahela they make up nearly one-half the thickness of the forma- 
tion. This is illustrated by a diamond-drill hole near Bissell, which 
reports between the Waynesburg and Pittsburg coals 130 feet of lime- 
stone, being over 41 per cent of the total thickness of the beds. This 
is due largely to the great thickness of the Benwood limestone. In 
the Washington formation the amount of limestone is less, but at 
least three beds reach thicknesses of 20 to 30 feet. 
USES. 
Certain impure limestones containing large percentages of silica, 
alumina, and iron oxide are capable of forming cement when burned 
in a kiln at a comparatively low temperature. Cements made in this 
manner are known as natural cements and are of much poorer quality 
than Portland cement, which is generally made from the purer lime- 
stones. For natural cement a limestone containing as high as 15 to 40 
per cent of impurities is necessary. This kind of cement has been 
made in the past in southwestern Pennsylvania to a limited extent, 
principally for use in the construction of locks on Monongahela River. 
The bed used for that purpose was a part of the Benwood limestone. 
Portland cement," which has for years surpassed all other kinds 
of cement in its value and use, requires a purer limestone. The pro- 
portion of magnesium carbonate (MgC0 3 ), especially, must be low — 
preferably less than 3 per cent. In general the amount of calcium 
carbonate (CaC0 3 ) must be at least 75 per cent, and the remainder 
(20 per cent) includes silica (S0 2 ), alumina (A1 2 3 ), and iron oxide 
(Fe 2 3 ). 
The principal use of limestones in this region at the present time 
is for road material, several quarries for this purpose being situated 
in the vicinity of Washington. The limestones seem also to be suited 
for railroad ballast, and when nearly pure carbonate of lime they 
may be used for flux in iron smelting. One of the most widespread 
uses of limestone in Pennsylvania has been for fertilizer, and for 
this purpose almost any except the most impure limes are suited. In 
Washington County, however, the great abundance of limestone beds 
a A detailed discussion of the Portland cement industry and cement-making limestones of the tinted 
States is given by E. C. Eckel in Bull. U. S. Geol. Survey No. 243, 1905. 
