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I-5 <br />,• <br />ROCK TYPE DEFINITIONS AND DESCRIPTIONS <br />SANDSTONE <br />Any clastic rock composed of grains easily visible to the <br />naked eye which range in size from 62u to 2000u. Cementing <br />materials are often calcareous and induration is quite variable. <br />Carbonaceous material is generally absent, but may occur as <br />long, thin filaments along bedding. <br />SILTSTONE <br />Any clastic rock composed of grains barely perceptable <br />or invisible to the naked eye which range in size from 4u to <br />62u. The silt size particles may generally be discerned with <br />the aid of a lOX hand lens. These rocks are gritty distinguish- <br />ing them from claystones. Induration is generally greater than <br />in typical sandstones and siltstones generally represent the <br />mechanically strongest group of rocks derived from coal pro- <br />ducing environments. Carbonaceous content is quite variable. <br />. Many of the rocks are weak in the bedding direction and subject <br />to slaking and water induced deterioration. <br />CLAYSTONE <br />Any clastic rock composed of clay-sized clastic materials <br />which range in size up to 4u• The clay-sized particles are <br />generally too small to be discerned with the aid of a 10X hand <br />lens. This group usually represents the mechanically weakest <br />and most anisotropic clastic rock type found in coal producing <br />environments. Carbonaceous content is generally highest in <br />this clastic group, but may be quite variable. These rocks <br />are generally friable and fissile and thus, very weak in the <br />bedding direction. claystones are generally subject to slak- <br />ing and rapid water induced disintegration. <br />MUDSTONE <br />Any clastic rock composed of an indefinite mixture of clay, <br />silt and sand particles of which the proportions vary from <br />place to place so that a more precise term is not possible. <br />. Organized stratification is generally absent and the rocks are <br />generally weak and exhibit high mechanical anisotropy. <br />$EEGMILLEP IMENNATIOIAAL <br />