152 The American Geologist. March, 1902 
are calcined after mixture (which mixture may be either nat- 
ural or artificial). In this sub-group fall the hydraulic limes, 
and the natural and Portland cements. The pozzuolanic cem- 
ents, which fall into the second sub-group, are formed by sim- 
ple mixture, without subsequent calcination, of materials of 
proper composition, one of the materials being invariably 
slaked lime, and the other a silico-aluminous material. 
Hydraulic Limes. The presence of a certain amount of 
silica in a limestone gives hydraulic properties to the lime pro- 
duced by its calcination. Theoretically the proper composition 
for a hydraulic limestone should be calcium carbonate 86.8%, — 
silica 13.2%. The hydraulic limestones in actual use, however, 
usually carry a much higher silica percentage, reaching at 
times to 25% ; while alumina and iron are commonly present in 
quantities which may be as high as 6%. Though not positively 
detrimental to the quality of the hydraulic lime, it is probable 
that both alumina and iron may be regarded as inert, and there- 
fore negatively harmful. The lime content of the limestones 
commonly used varies from 55% to 65%. 
After burning, water is added (usually by sprinkling) to 
slake the portion of the product consisting of free lime which 
must result from burning a limestone of the composition noted 
above. The slaking of this part of the product suffices to dis- 
integrate the entire mass, so that crushing is unnecessary. 
The hydraulic limes are, relatively to the cements proper, 
feebly hydraulic. The abundance of materials suitable for the 
manufacture of natural cements has prevented the manufac- 
ture of hydraulic lime in the United States, though in Europe 
the industry is of considerable importance. 
It seems probable that some hydraulic limes low in silica 
depend for their hydraulicity entirely upon the aluminates 
found during burning, but they are of little engineering value, 
or commercial importance. 
Natural Cements, This class, which is retained because of 
engineering necessities, is very heterogeneous. As commonly 
defined, it includes all cements manufactured by burning, at 
comparatively low temperatures (i. e., below the clinkering 
point) limestones containing sufficient silica, alumina or mag- 
nesia to give a hydraulic product. The difficulty in presenting 
a comprehensive definition for the group arises from the fact 
