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)



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