Country rock (geology) in geology when the term "country rock" is used include: When describing a pluton or dike, the igneous rock can be described as intruding the surrounding "country rock", the rock into which the pluton has intruded. When country rock is intruded by dyke, it is called "discordant intrusion," while intrusion by a sill indicates a "sub-parallel" or "concordant intrusion". Most intrusions into country rock are via magma. Usually, country rock is intruded by an igneous body of rock which formed when magma forced upward through fractures, or melted through overlying rock. Magma then cooled into solid rock, different from the
Obsidian manufacture cutting and piercing tools and it has been used experimentally as surgical scalpel blades. The translation into English of "Natural History" written by Pliny the Elder of Rome shows a few sentences on the subject of a volcanic glass called obsidian ("lapis obsidianus"), discovered in Ethiopia by Obsidius, a Roman explorer. Obsidian is the rock formed as a result of quickly cooled lava, which is the parent material. Extrusive formation of obsidian may occur when felsic lava cools rapidly at the edges of a felsic lava flow or volcanic dome or when lava cools during sudden contact with water