HORNFELS
Hornfels are dense, compact, fine-grained, contact
metamorphic rocks. They typically form as the result of contact
metamorphism of shales or basalts. Some hornfels have distinctive rounded
patches of more coarsely-crystalline material (see dark greenish spheroidal
patches in the sample shown below) - such rocks are called spotted hornfels.
Spotted hornfels (5.6 cm across)