Hornfels is a generic name for contact metamorphic rocks that are non-foliated and too fine-grained to allow ready mineral identification. They are often dark, kind of cruddy looking, and would be difficult to recognize if someone handed you one out of the blue. However, if you spot a dike and some resistant rocks along the dike, you would probably be safe in using the term 'hornfels'.

The dike runs vertically through the picture. The hornfels is best seen to the left of the dike near the students. A few scraps are also seen in various spots to the right of the dike.
The dike is the big blocky outcrop running vertically up the hillside.

The rocks on either side of this dike are hornfels. To see where the hornfels ends and the host rocks begin, you'd have to be able to walk around and examine the host rocks away from the dike!

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