380 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
make are being extensively manufactured and are of equal or even' 
superior quality to the foreign-made cements. During the year 1890 
it is estimated that 0,000,000 barrels of Portland cement have been 
used, of which four-fifths were of home manufacture. In spite of 
this fact, on account of the failure to find suitable clays at the various 
localities and also the difficulty of obtaining fuel, it is impracticable 
to manufacture Portland cement at the dam sites. 
It is important, if Portland cement is used at the localities under 
consideration, that the specifications should require it to be ground 
so fine that not over 1 per cent residue shall remain on a 100-mesh 
screen. 
Reliable sand cement can be made from the quartzite at San Carlos 
or the pearlite at the Buttes dam site by grinding with Colton (Port- 
land) cement. A saving will result at the San Carlos dam site of $1.64 
per cubic yard by making sand cement on the ground, this being 
exclusive of the cost of the plant for grinding. 
