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.
Granites are usually low in iron-magnesium minerals and contain abundant quartz and feldspar.
Dikes and sills are intrusive igneous rocks, injected in the molten state into previously solidified rocks. Such rocks may result from the activity of calcite-precipitating organisms, but most limestone is probably the result of inorganic chemical precipitation from sea water. Chalk is a form of limestone. The original rocks might have been sedimentary or igneous, or even older metamorphic rocks. Micas are a group of silicate minerals that have a platy or sheet-like structure.