Parts of the teeth include: 1  Enamel: The hardest, white outer part of the tooth. Enamel is mostly made of calcium phosphate, a rock-hard mineral. 2  Dentine: A layer underlying the enamel. Dentine is made of living cells, which secrete a hard mineral substance.  Pulp: The softer, living inner structure of teeth.
Though they vary significantly, transportation costs accounted for 40% of the average overall cost of coal delivered at electric power plants in 2010. On average, nominal U.S. rail rates for shipping coal grew from $11.83 to $17.25 per short ton from 2001 to 2010.
U.S. Travel Answer Sheet. U.S. Travel & Tourism Overview. Direct spending by resident and international travelers in the U.S. averaged $2.7 billion a day, $113.1 million an hour, $1.9 million a minute and $31,400 a second. Direct spending on leisure travel by domestic and international travelers totaled $683.1 billion in 2016.