Types of rocks and fossils

Geologists in Darwin’s day were familiar with the diversity of types of rocks, but they were only beginning to appreciate the vast time scales over which geological events occurred. They had yet to understand the dynamic nature of the Earth’s crust, and they lacked modern understanding of how these types of rocks formed:

• Igneous: Igneous rocks are of volcanic origin. They form when molten lava from volcanoes cools and solidifies. Basalt and obsidian are examples of igneous rocks.
• Sedimentary: Sedimentary rocks are formed by the gradual deposits of sediments. Sandstone is an example of sedimentary rock.
• Metamorphic: Metamorphic rocks are rocks of any origin that have been subjected to the extreme stresses and temperatures caused by the folding and crushing of the Earth’s crust.

Understanding these rock types helps biologists understand the fossil record. Fossils are found only in sedimentary rocks. The molten lava that bubbles up from beneath the Earth’s crust
during a volcanic eruption doesn’t contain any fossils (whatever had been there would have melted in the molten rock). Metamorphic rocks — even those of sedimentary-rock origin — don’t contain fossils, because the extreme temperatures and pressures that converted the rock from sedimentary to metamorphic would have destroyed whatever fossils may have been there.