For this reason many limestone gravestones in northeastern US are becoming illegible because acid rain is eroding their carvings. See also Amethyst Galleries. Volcanic ash and lava flows solidify to form extrusive rocks. More or less synonymous with volcanic. fault Figure Faults are surfaces of breakage in rocks, along which some movement has taken place. The movement may be measured in millimeters or miles.
Feldspars are aluminum-rich silicate minerals that are very common in rocks like granite.
As a group, feldspars are the most common minerals in the Earth's crust. With details of stratigraphy and fossils for most of the geologic periods, and nice links to more information. Commonly, though by no means always, the minerals are dominantly quartz and feldspar, making a quartzo-feldspathic gneiss. The planar compositional bands define the rock's foliation. A common igneous rock that crystallizes slowly, within the earth, so that mineral crystals have time to grow to relatively large size.