harmful; however, in larger quantities it sets up planes of structural 

 weakness and provides a starting point for disintegration. Granite occurs 

 mainly in large bodies, known as batholiths, which are exposed over many- 

 square miles. Batholiths commonly consist of numerous individual bodies of 

 various granitic rock types, with contrasting colors, textures, and mineral 

 composition. 



While granites vary widely in texture and appearance, most of them are 

 dense and have a porosity of less than 1 percent. Granite spalls badly 

 under the combined effect of fire and water, so it is not particularly 

 resistant to fire. Most unweathered granitic stones are hard, strong, 

 tough, and resistant to abrasion, impact, and chemical attack. The average 

 unit weights range from approximately 24.3 to 27.5 kilonewtons per cubic 

 meter (155 to 175 pounds per cubic foot) . These properties make granitic 

 stones well suited to use as riprap and quarrystone armor units. 



c Basalt and Related Stone . In commercial usage, the term "basalt" 

 is applied to any of the dense, fine-grained, dark-gray or black volcanic 

 stone. The term ordinarily includes stone types that geologists classify 

 as dacite, andesite, basalt, trachyte, or latite. Basaltic rock has 

 solidified by the cooling of lava, either as flows on the Earth's surface, 

 or as shallow intrusive bodies beneath the surface. It is composed primarily 

 of feldspar and ferromagnesian minerals in crystals that range in size from 

 submicroscopic to clearly visible. Commonly, an appreciable percentage of 

 glassy material is present. Some effusive basalt is vesicular and the 

 vesicles may have become filled with potentially reactive substances such 

 as opaline, silica or zeolites which render the rock unfit for use as 

 aggregate. 



Basalts are one of the heaviest stones with an average specific gravity 

 of 2.9 to 3.2 and average unit weights of 28.3 to 31.4 kilonewtons per 

 cubic meter (180 to 200 pounds per cubic foot)); however, in certain areas 

 they may contain many small cavities (vesicles) which result in stone with 

 low densitities. Basaltic stones are characteristically hard, tough, and 

 durable, so they are well suited for use as aggregate, riprap and quarry- 

 stone armor units . 



d. Carbonate Stone . Carbonate stones are broadly divided by geologists 

 into (1) limestone, which consists almost entirely of calcite (CaC0 3 ); (2) 

 dolomite, which consists mainly of the mineral dolomite (CaC03 . MgC0 3 ); and 

 (3) marble, which is the metamorphosed crystalline equivalent of either 

 type. All gradations exist between limestone and dolomite and between very 

 fine-grained and very coarse-grained material. 



In the stone industry the term limestone is applied to many types of 

 rock that contain a high percentage of calcium carbonate, although large 

 proportions of other substances also may be present. Such substances 

 include siderite (FeC0 3 ), magnesite (MgC0 3 ), and rhodochrosite (MnC0 3 ) . 

 They also commonly contain clay, silt, and sand grains. A high percentage 

 of clay commonly weakens carbonate rock, making it unfit for use as stone. 

 A high content of sand grains or silica may harden carbonate rock. Marble 

 is similar to limestone chemically, but has been subjected to a metamorphic 

 process which has made it more crystalline in structure, harder, and 

 better able to hold a polish. 



24 



