STATE GEOLOGIST. D3 
portant factor in the development of our country during the greater part 
of the nineteenth century. They have been especially important during 
the canal-building period, during the construction of the American canal 
systems, the Erie canal of New York, the Pennsylvania and Ohio canals. 
In the construction of all these waterways hydraulic limestone was 
found at many places; large as well as smaller local deposits were thus 
discovered and used. 
From what we now know of the nature of limestone we can realize 
without difficulty that these stones were apt to be of all sorts of com- 
positions, high or low in clay substance. When they contain 75 to 80 
per cent. of calcium carbonate and 25 to 20 per cent. of clay, they are 
called hydraulic limes, while those higher in clays come under the term 
Roman cement, containing 50 to 70 per cent. of calcium carbonate and 
50 to 30 per cent. of clay. Both may contain varying amounts of 
magnesia. Physically, these materials may vary enormously, since they 
may be soft or hard, like clay in appearance, or compact as the densest 
varieties of stone. The harder, more rock-like hydraulic materials are 
preferred, since in manufacture it is desirable to work with larger lumps 
which are not friable, so that they may be stacked up in the kiln without 
impairing the draft and in order that the heat may pass through the mass 
uniformly. 
We can, therefore, classify the natural hydraulic cements as follows: 
ito” JebyGbeeneuhie manss5 5 y alo's oe E CaeeRcOus: 
| Magnesian. 
2a Noman, Cements stoner): Caos 
2 Magnesian. 
The practical distinction between the hydraulic limes and the Roman 
cements is that the former after burning slake by themselves and evolve 
considerable heat, while the latter, though quite soft, must be ground to 
a powder and evolve less heat of hydration on slaking. In either class 
the magnesium carbonate may be anything from a trace to about four- 
fifths of the calcium carbonate, that is, for a content of 43.5 per cent. 
of calcium carbonate we may have 36.5 per cent. of magnesium car- 
bonate, the sum of the carbonates being the maximum content of carbon- 
ates allowable in a hydraulic lime. 
Owing to the fact that hydraulic lime is such an extremely irregular 
source of cement making material, it need not be considered in detail 
here, but we shall discuss only the Roman cements as representing the 
general type of natural cements. 
In the following table some typical analyses of Roman cements are 
given: 
