<P> The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for limestone exposure in the workplace as 15 mg / m total exposure and 5 mg / m respiratory exposure over an 8 - hour workday . The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 10 mg / m total exposure and 5 mg / m respiratory exposure over an 8 - hour workday . </P> <P> The cyanobacterium Hyella balani can bore through limestone; as can the green alga Eugamantia sacculata and the fungus Ostracolaba implexa . </P> <Ul> <Li> <P> A stratigraphic section of Ordovician limestone exposed in central Tennessee, U.S. The less - resistant and thinner beds are composed of shale . The vertical lines are drill holes for explosives used during road construction . </P> </Li> <Li> <P> Thin - section view of a Middle Jurassic limestone in southern Utah, U.S. The round grains are ooids; the largest is 1.2 mm in diameter . This limestone is an oosparite . </P> </Li> <Li> <P> Photo and etched section of a sample of fossiliferous limestone from the Kope Formation (Upper Ordovician) near Cincinnati, Ohio, U.S. </P> </Li> <Li> <P> Biosparite limestone of the Brassfield Formation (Lower Silurian) near Fairborn, Ohio, U.S., showing grains mainly composed of crinoid fragments . </P> </Li> <Li> <P> A concretionary nodular (septarian) limestone at Jinshitan Coastal National Geopark, Dalian, China . </P> </Li> </Ul> <Li> <P> A stratigraphic section of Ordovician limestone exposed in central Tennessee, U.S. The less - resistant and thinner beds are composed of shale . The vertical lines are drill holes for explosives used during road construction . </P> </Li>

The two main types of rock in nigeria