STATE GEOLOGIST. 193 
This means the rejection of unsuited rock based on chemical examina- 
tion, uniform burning, and fine grinding, together with any other improve- 
ments like the addition of dry slaked lime, whenever found beneficial. A 
great deal of the prejudice against natural cements is based on erroneous 
conceptions and they are barred frequently from fields in which by virtue 
of their cheapness they might be used with economy. 
In regard to the strength of Roman cements, the following figures 
might be cited. ‘Tetmayer found the average specific gravity of Swiss 
Roman cements to be 2.98, and their average strengths as given in table 
below. These cements are principally calcareous. 
Tensile and Crushing Strength of Swiss Roman Cements, According to 
Tetmayer. 
3 ee ae 
E | 58 ee 
3 g ae ae 
i @ Felis | a5 
4 g 5 3 8 
= = B.8 O.8 
Neat 28 days 203 2,347 
“ g4 356 Beri 
ce 210 401 5,600 
c 365 416 6,676 
ile 2 28 days - | 190 aly 
“ g4. Bid 2,884 
ce 910“ | 396 4,025 
c 365° | 407 4,517 
Standard Test.—With reference to American natural cements, the 
following minimum and maximum tensile strengths per square inch have 
been suggested by a committee of the American Society of Civil En- 
gineers : 
Neat Cement—One day; one hour, or until set, in air, the rest of phe 
24 hours in water, from 40 to 80 pounds. 
One week; one day in air, six days in water, from 60 pounds to 100 
pounds. 
One month (28 days) ; one day in air, 27 days in water, from 100 to 
150 pounds. 
One year; one day in air, the remainder in water, from 300 to 400 
pounds. 
Sand Mixtures—1 cement: 1 sand, by weight. 
18—S. G. Bull. 3. 
