eckel.] LIMESTONES. 31 
CEMENT ROCK OF THE LEHIGH DISTRICT. 
The Lehigh district of the cement trade comprises parts of Berks, 
Lehigh, and Northampton counties, Pa., and of Warren County, N. J. 
Within this relatively small area are located about i^» Portland-cement 
mills, which produce a little over two-thirds of the entire American 
output. As deposits of the cement rock used by these plants extend 
far beyond the present Lehigh district, a marked extension of the 
district will probably take place as the need for larger supplies of 
paw material becomes more apparent. 
The "cement rock" of the Lehigh district is a highly argillaceous 
limestone of Trenton (lower Silurian) age. The formation is about 
300 feet thick in this area. The rock is very dark gray, and usually 
has a slaty fracture. In composition it ranges from about <>0 per cent 
lime carbonate with 30 per cent clayey material, up to 80 per cent 
lime carbonate with 15 per cent of silica, alumina, and iron. The 
lower beds of the formation always contain more lime carbonate than 
those above. The content of magnesium carbonate in these cement 
rocks is alwa} T s high, as Portland cement material goes, ranging from 
3 to 6 per cent. 
Near and in some cases immediately beneath these cement beds 
are beds of purer limestone, containing from 85 to 96 per cent lime 
carbonate. The usual practice in the Pennsylvania and New Jersey 
plants has been, therefore, to mix a relatively small amount of this 
purer limestone with the low lime "cement rock" in such proportions 
as to give a cement mixture of proper composition. 
The economic and technologic advantages of using such a combina- 
tion of materials are very evident. Both the pure limestone and the 
;cement rock, particularly the latter, can be quarried very easily and 
cheaply. As quarried they carry but little water, so that the expense 
of drying them is slight. The fact that about four-fifths of the 
cement mixture will be made up of a natural cement rock permits 
coarser grinding of the raw mixture than would be permissible in 
plants using pure limestone or marl with clay. This point is more 
fully explained on page 47. When natural cement rock is used as 
part of the mixture less fuel is probably necessary to clinker the mix- 
ture than when pure limestone is mixed with clay. 
Such mixtures of argillaceous limestone or "cement rock" with a 
small amount of pure limestone evidently possess important advan- 
tages over mixtures of pure hard limestone or marl with clay. They 
are, on the other hand, less advantageous as cement materials than the 
chalky limestones discussed on pages 33-34. 
The analyses in the table below are fairly representative of the 
materials employed in the Lehigh district. The first four analyses 
are of "cement rock," the last two are of the purer limestone used for 
mixing with it. 
