ECKEL.] 
SLATE. 
39 
Commercial considerations in connection with the slate industry, 
however, make slate a very important possible source of cement mate- 
rial. Good rooting slate is a relatively scarce and commands a good 
price when found. In the preparation of rooting slate for the market 
so much material is lost during sawing, splitting, etc., that only 
about 10 to 25 per cent of the amount quarried is salable as slate. 
The remaining 75 to 90 per cent is of no service to the shite miner. 
It is sent to the dump heap, and is a continual source of trouble and" 
expense. This very material, however, as can be seen from the 
analyses quoted below, is often admirable for use, in connection with 
limestone, in a Portland-cement mixture. As it is a waste product it 
could be obtained very cheaply by the cement manufacturer. 
Composition of American roofing slates. 
Silica (Si0 2 ) 
Alumina ( A1 2 ?> ) 
Iron oxide (FeO, Fe 2 3 ) 
Lime (CaO) 
Magnesia (MgO) 
Alkalies (K 2 0, Na.,0)... 
Ferrous sulphide ( FeS 2 ) . 
Carbon dioxide (C0 2 )... 
Water of combination. . . 
Moisture below 110°C . . 
Maximum. 
liS. 62 
24.71 
10.66 
5. 23 
6.43 
s. us 
Average. 
60.64 
IS. 05 
6.87 
1.54 
2. 60 
4.74 
.38 
1.47 
3. 51 
.62 
Minimum. 
54. 05 
1). 77 
2. IS 
.00 
. 12 
1.93 
VALUE OF DEPOSITS OF CEMENT MATERIALS. 
The determination of the possible value for Portland-cement manu- 
facture of a deposit of raw material is a complex problem, depending 
upon a number of distinct factors, the more important of which are as 
follows: (1) Chemical composition, (2) physical character, (3) amount 
available, (4) location with respect to transportation routes, (5) loca- 
tion with respect to fuel supplies, (6) location with respect to markets. 
Ignorance of the respective importance of these factors frequently 
leads to an overestimate of the value of a deposit of raw material. 
Their effects may be briefly stated, as follows: 
(1) Chemical composition. — The raw material must be of correct 
chemical composition for use as a cement material. This implies that 
the material, if a limestone, must contain as small a percentage as 
possible of magnesium carbonate. Under present conditions, 5 or 6 
per cent is the maximum permissible. Free silica, in the form of 
chert, flint, or sand, must be absent, or present only in small quan- 
