LIMES AND CEMENTS.                  677
rived at. Quite various compositions have been found to give good
results. In general terms, it may doubtless safely be said to be due to
the formation, chemically, of an aluminous-lime-magnesian silicate.
Hydraulic limestones are to be sought among the impure magne-
sian limestones and highly calcareous shales. Some of these are
known to possess hydraulic properties in a measurable degree, and
the constitution of others makes it highly probable that they do
also. There is little reason to expect, however, that a stratum pos-
sessing an excellence equal to that now utilized near Milwaukee will
be found, though the variation in character of hydraulic cements
renders a judgment in advance of actual tests of uncertain value.
The Milwaukee cement is quite fully described in Volume II, to
which reference is invited. Experience, so far as I am informed,
justifies the high estimates which the early tests encouraged.