Geology 101 Redux

Variolite "Cross-section" - Elwha River

One of my favorite finds from my search for variolites on Sunday was this fellow. About 6" inches long, it shows the distinctive feldspar "dots" of all variolites, but grading dramatically in size from the bottom to the top.  Varioites are believed to form inside "pillow lavas" or basaltic lava that erupts underwater, forming unmistakable round structures visible in many NW roadcuts (especially on the Olympic peninsula). My sources tell me that the smaller dots form as the edges of the pillows, which cool more quickly, while the larger dots grow in the slower-cooled, better-insulated interior of the rock.

This makes sense to me, but whatever the explanation, I thought this sample was particularly cool...