The primary emissions responsible for acid deposition are sulfur dioxide (SO2) and oxides of nitrogen (NOx) from the combustion of coal, oil and natural gas. The combustion compounds are transformed into sulfuric and nitric acid and transported downwind before they are deposited.
Index fossils (also known as guide fossils, indicator fossils or zone fossils) are fossils used to define and identify geologic periods (or faunal stages).
