What is Geochemistry?
"The study of the distribution and amounts of the chemical elements in minerals, ores, rocks, soils, water and the atmosphere." Glossary of Geology, 4th ed., American Geological Institute.
What is Petrology?
"That branch of geology dealing with the origin, occurrence, structure, and history of rocks, esp. igneous and metamorphic rocks." Glossary of Geology, 4th ed., American Geological Institute.