very resistant to chemical and physical weathering. These properties make 

 it desirable as crushed and broken stone for riprap, but generally not for 

 quarrystone armor units. 



(5) Shale . Shale is a very fine-grained, thinly bedded sedi- 

 mentary rock composed mostly of clay-size and silt-size particles. Pre- 

 Mesozoic shales are commonly well indurated, if not metamorphosed. Most 

 Mesozoic and Tertiary shales are moderately to poorly indurated. Most 

 types of shale are too weak to be suited to the ordinary uses of crushed 

 and broken stone. 



(6) Slate . This stone is a thinly foliated metamorphic rock 

 composed essentially of muscovite (sericite) , quartz, and graphite, all in 

 grains of microscopic or submicroscopic size. Slate is formed by compaction 

 and partial recrystallization of shale, and is commonly dark colored and 

 moderately hard. Slate is desired mainly for use as dimension stone. 

 Because slate has been subjected to intense pressure during formation, it 

 has a low porosity and, consequently, a high strength. Its modulus of 

 rupture is relatively high, and it is also resistant to weathering and to 

 mechanical abrasion. The average unit weight ranges from approximately 

 26.7 to 28.3 kilonewtons per cubic meter (170 to 180 pounds per cubic 



foot) . 



(7) Tuff . The term "tuff" includes pyroclastic volcanic types, 

 most of which would be classed as rhyolite or dacite tuffs or tuffaceous 

 sediments. Most tuffaceous rocks are only moderately hard, although on 

 exposure to air they commonly harden appreciably. Because of its softness, 

 tuffaceous rock is unsuited to most uses of crushed and broken stone. 



(8) Coral . In southeastern United States and certain Pacific 

 Island areas, it is often necessary to make use of coralline limestones for 

 coastal construction, since more durable stone (such as granite or basalt) 

 is unavailable. These materials are partially recrystallized coral forma- 

 tions which have sufficient resistance to breakdown and erosion to be 

 acceptable for breakwater or revetment construction (Fig. 1). They are 

 less resistant to mechanical breakage than denser, harder stone and there- 

 fore, require special care in blasting and handling. However, it has been 

 possible to produce large stones 89 to 267 kilonewtons (10 to 30 short 

 tons) from coralline limestones. 



2. Categories of Stone Size and Gradation . 



a. General . The category of crushed and broken stone includes all 

 quarried stone that is not cut or shaped to specified dimensions. It 

 ranges in size from granite blocks weighing 222 to 267 kilonewtons (25 to 

 30 short tons) used as quarrystone armor units, to ground shale of very 

 small size to smaller than 200 mesh (0.075 millimeter) used as insecticide 

 carrier. The larger categories of crushed or broken stone (greater than 

 75 millimeters or 3 inches) are generally used in coastal construction. 



b. Fill . Most fill material is natural earth obtained as surplus from 

 excavation or from borrow pits, and would not be classed as stone. Crushed 

 stone is used for special types of fill, such as the cores of dikes or 

 jetties. Only the least expensive grades of crushed stone, commonly 



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