An porphyritic* form of Rhyolite found only at the very base of Palisade Head on Lake Superior, a place my climbing career has steered me toward many times.

*”Porphyritic” means that while the rock was still molten magma — but getting near the surface and thus considerably cooled down — certain minerals like quartz and feldspar began forming crystals. This magma soon spewed out and formed solid lava. The stuff that had been liquid cooled into a fine-grained rock interspersed with the larger, prominent crystals seen as little blocky structures in the illustration. Not to be confused with the lighter horizontal band, but that’s another story about flow-banding.