Leona E. Adkins

This rock would be called a monzonite, which has a darker gray color than the marker at Stop 3. But there is another key difference between this marker and the previous one. They have different sized crystals. Some igneous rocks have bigger crystals than others. (Image 2)

How big the crystals grow in an igneous rock is a function of how fast the magma cools. If the magma cools quickly, the crystals will be small in size. If the magma cools slowly, the crystals will be large in size.

Geologists use different terms to describe the general size of the crystals in an igneous rock. Aphanitic rocks have very small crystals, so small that they are not visible to the naked eye (although you might pick out one occasionally). Aphanitic rocks are not commonly used as cemetery markers because they aren’t very durable. Most of the igneous rocks in Parsons Cemetery are phaneritic, meaning the crystals can be seen with the naked eye. Pegmatitic rocks have crystals which are larger than a dime. Porphyritic rocks have two groups of distinctly different-sized crystals. (Image 3).