THE MINERAL INDUSTRY OF MARYLAND 
 
 
Table 4.—Employment and injury experience in the mineral industries

393 
Year and industry 
Average 
 
 
men 
 
 
working 
 
 
daily 
Days 
 
 
active 
Man- 
 
 
days 
 
 
worked 
 
 
(thou- 
sands) 
Man- 
hours 
worked 
(thou- 
sands) 
Number of injuries 
  
Fatal Non- 
fatal 
Injury rates per million man-hours 
—        Fre- Severity quency 
1966: 
Coal and peat               
Nonmetal                  
Sandandgravel             
Stone                     
 
Total'                  
 
1967: P 
Coal and peat                
Nonmetal                   
Sand and gravel              
Stone                      
 
Total'                    
372 
364 
822 
1,204 
212 
254 
267 
262 
79 
93 
219 
316 
638 
766 
1,917 
2,677 
11 
25 
43 
63 
 17.25 365 32.64 8,461 22.43 2,968 23.54 2,088 
 
2,762 
256 
707 
5,997 
142 
 23.68 3,000 
 
420 
430 
825 
1,095 
211 
239 
263 
270 
88 
103 
217 
296 
726 
847 
1,878 
2,510 
 1 9 - - - - 36 2 45  57 
13.78 8,61542.52 60425.03 7,26722.71 453 
 
2,770 
254 
704 
5,960 
3 147 
25.17 3,615 
P Preliminary. 
' Data may not add to total shown because of independent rounding. 
 
REVIEW BY MINERAL COMMODITIES 
 
NONMETALS 
 
 Cement.—A slight decline in cement output from 1966 levels reflected
lessening in construction activity. Carroll County was the State's leading
cement producer, but plants in Frederick and Washington Counties also contributed
significantly to the State's total output. Portland cement represented about
90 percent of the total production. A majority of the cement output was used
for making ready-mixed concrete for road and building construction. 
 
 Clays.—Clays were produced in nine of the State's 23 counties. Nearly
all of the clay was classified as miscellaneous clay, which was used chiefly
for making building brick, lightweight aggregate, and portland cement. Small
quantities of fire clay were produced in Allegany and Harford Counties, while
ball clay production was reported from Baltimore County. Fire clay and stoneware
clay were used in manufacturing refractories, building brick, and vitrified
sewer pipe, while ball clay was sold for use in stoneware, refractories,
and tile. The unit value of all clays remained nearly constant, but production
increased by 17 percent. 
 
 Gem Stones.—Small quantities of semiprecious gem stones were collected
by dealers and amateur collectors. The 
value was estimated to be only a few thousand dollars. 
 
 Lime.—Production of lime was limited to Frederick County, where three
plants produced quicklime and hydrated lime. The total tonnage of both types
of lime was slightly less than that of 1966. 
 
 Marl, Greensancl.—Greensand marl was mined in Calvert County. The
output was marketed for use in the manufacture of fertilizer, and as a soil
conditioner. 
 
 Perlite.—Expanded perlite was produced in Baltimore and Prince Georges
Counties by three companies each operating one plant. The product was sold
for a wide variety of purposes, including use in building plaster, insulating
material, concrete aggregate, as a filtering medium, and as a soil conditioner.

 
 Potassium Salts.—A Washington County cement plant produced low-grade
potassium sulfate as a byproduct and sold it for agricultural uses. 
 
 Sand and Gravel.—Total production (commercial and Government-and-contractor)
of sand and gravel declined 15 percent from that of 1966; the average unit
value of $1.38 per ton was slightly higher. Sixty-four companies conducted
operations at 80 pits. There were 54 stationary plants, eight portable plants,