bckel.] NATURAL CEMENTS. 385 
lessly or ignorantly laid on the fact that many of the best-known nat- 
ural cements carry large percentages of magnesia, but magnesia (in 
natural cements at least) may be regarded as being almost exactly 
interchangeable with lime, so far as the hydraulic properties of the 
product are concerned. The presence of magnesium carbonate in a 
natural-cement rock is then merely incidental, while the silica, alumina, 
and iron oxide are essential. The 30 per cent or so of magnesium car- 
bonate which occurs in the cement rock of the Rosendale district, New 
York, could be replaced by an equal amount of lime carbonate and the 
'burnt stone would still give a hydraulic product. If, however, the 
clayey portion (silica, alumina, and iron oxide) of the Rosendale rock 
jcould be removed, leaving only the magnesium and lime carbonates, 
the burnt rock would lose all of its hydraulic properties and would 
yield simply a magnesian lime. 
This point has been emphasized because many writers on the sub- 
ject have either explicitly stated or implied that it is the magnesian 
carbonate of the Rosendale, Akron, Louisville, IJtica, and Milwaukee 
rocks that causes them to yield a natural cement on burning. 
Since within very wide limits of composition any clayey limestone 
will give a natural cement on burning, it can readily be seen that sat- 
isfactory natural-cement materials must be widely distributed and of 
[common occurrence. Hardly a State is entirely without limestones 
ufficiently clayey to be available for natural-cement manufacture. 
he sudden rise of the American Portland-cement industry, however, 
ias acted to prevent any great expansion of the natural-cement indus- 
ry. It would be difficult to place a new natural cement on the market 
n the face of competition from both Portland cement and from the 
older and well-established brands of natural cement. Such new nat- 
iral-cement plants as have been started within recent years have 
nostly been located in old natural-cement districts, where the accu- 
mulated reputation of the district would help to introduce the new 
>rand. The only exceptions to this rule, indeed, were the Pembina 
)lant in North Dakota, the Rossville plant in Georgia, and a plant in 
he State of Washington. Of these, the Pembina plant was estab- 
ished with the intention of making Portland cement, but the raw ma- 
terials soon proved to be unsuitable and the plant was converted. The 
blant in Washington is located in an area where any kind of cement is 
readily salable. The Rossville plant was built by an Akron, N. Y., 
;ement manufacturer to utilize a peculiarly satisfactory natural-cement 
|ock. 
METHODS OF MANUFACTURE, 
The manufacturing methods at a natural cement plant are of the 
implest kind, including merely the burning of the cement rod; and 
he pulverizing of the product. 
