|What is it?
A porphyritic volcanic rock. Such rocks are common in the American
west (this rock was found in Utah). A porphyritic rock is one where
there are 2 distinct sizes of mineral crystals, one very small and
the other large. This aspect is easier to see on the the other
side of the rock, which is a sawn face. Nos. 086
& 129 are also porphyritic volcanic rocks