Mr. Obama rarely plays with other politicians - though he has played 7 times with Vice President Biden. In a fence-mending gesture, Mr. Obama invited then-House Speaker John Boehner to play on one occasion in June 2011 along with Gov. John Kasich, both Ohio Republicans.
Here are some ways to classify coal (by grouping it with similar types of rocks): Because slag formed from lava-like melted rock, it's sort of like an igneous rock -- but because humans made the melt, it's not a true igneous rock. So, we made up a new classification for slag: We call it a pseudo-igneous rock (pronounced SUE-doe ig-NEE-us).
Coal comes in different types, depending on the extent of compaction and alteration. Obviously, we define the term 'sediment' pretty broadly to classify coal! At the other end are a group of rocks called evaporites. They're rocks made of sediment produced by purely chemical reactions as water evaporates. Inland lakes or seas, like the Dead Sea, are environments where that can happen. The two common evaporite rocks are gypsum and salt. In the middle of this category are rocks made from sediment originally produced by either plants or animals living in water.