Granite, basalt, diabase, and obsidian are examples. Metamorphic rock is the kind that has undergone levels of heat and pressure above that at the earth's surface or its atmosphere. Metamorphic rock is usually foliated as the result of pressure and parallel cleavage planes are abundant. This rock may weather relatively rapidly or extremely slow. Schist, gneiss, and slate are examples.
Sedimentary rock is formed through the cementation of sediments under slight pressure.
The cementing agent may be a variety of substances such as silica, iron, calcium, etc. The sediments mat have been deposits of eroded material from any of the three types of rock. Their character is quite variable. Sandstone, shale, limestone, and conglomerate are examples of sedimentary rock. This type of rock is the most common found at the earth's surface, simply because the earth's crust has been through many repeated cycles of uplift followed by erosion and leveling during geologic history.