Basalt-porphyry. The large white crystals are phenocrysts of plagioclase feldspar
Basalt-obsidian
Plate 56
Amgydoloid
Plate 57
The north face of Scott’s Bluff, Neb., showing sedimentary sandstones above and clays below. The type of erosion is characteristic of arid regions. Courtesy of the U. S. Geological Survey