STATE GEOLOGIST. 183d 
any considerable decrease in tensile strength, indicating that as long as 
there is sufficient lime to change the quartz to “soluble” silica about the 
same hydraulicity is maintained. It cannot be said, therefore, that good 
Roman cements must correspond to a definite formula, as has been claimed 
by Jex, who lays down as the type formula a compound corresponding to 
8€aO3zSiO,Al1,O,; which, according to him, hydrates to 3CaSiO,5Ca 
(OH),2(Al,(OH),, thus resulting in 42.4 per cent. of meta-calcium sili- 
cate and 57.6 per cent. of calcium nd aluminum hydrates. 
4. The proper burning temperature of dolomitic Roman cements is 
about 950° C. and should be lower rather than higher. 
5. silicious dolomitic cements give rise to greater hydraulicity in 
every case than those containing pure clay substance as a base. 
6. Dolomitic cements show a greater hydraulicity than calcareous 
cements. Whether or not this is due to a reinforcement of the pozzuolane 
KeacwOlN apViuna ya SOhelmGcement)  sneaction. (cannot) bey said:  altesus 
Possible; thatmut wythel 24 ((Calvics|@Si@s. cements: ywith they 50 
kaolin 44 quartz clay base had been burnt more rapidly up 
to 1300° it would have shown a reconstruction of hydraulicity, owing 
to the formation of a vitrified magnesian Portland cement. None of the 
less basic cements, however, could have shown such a behavior. If it 
had been still more basic, corresponding, about, to 2.8(CaMg)OSiO, 
2(CaMg)OAI,O, we could predict the formation of a Portland cement 
and hence a great rise in hydraulic strength with certainty. 
Similarly, a Roman cement high in lime, approaching a tri-calcium 
silicate, would become by vitrification a Portland cement with great in- 
crease in strength. This is actually realized in the, high cal- 
cium natural cements of the Lehigh Valley in Pennsylvania. If the 
tensile strength of such basic cements, calcareous or magnesian,wereplot- 
ted co-ordinate with the temperature of burning, we would have a curve 
showing two minima and two maxima. The first minimum would of 
course be due to under burning, the first maximum would be reached by 
the proper maturing temperatures, 1000° and 950°, respectively; the sec- 
ond minimum would be due to too high a temperature while the body 
still remains in the earthy condition, but becomes dense, and the second 
maximum is the result of vitrification, that is, the production of more 
complex hydraulic compounds. 
Gilmore’s Work.—This fact was already realized by General Gil- 
more in his treatise on “Limes, Hydraulic Cements and Mortars,” 1870, 
where he gives, on page 158, curves bringing out this fact. He took 
Ulster county, N. Y., cement, burnt it at four stages represented by 
125 O45 respectively, where t stands for underburnt rock, 2 for properly 
