The Three Classes of Rocks

Class and Origin	Texture	Common Types
Igneous From the hot, molten magma which either comes to the surface as lava, or is cooled and solidified within the crust. The rate of cooling and the mineral content of the magma determine the kind of igneous rock. Rocks with visible mineral crystals result from slow cooling. The faster the magma cools, the smaller are the crystals.	Large crystals	PEGMATITE originates deep in the crust, forming very large crystals due to slow cooling.
	Small crystals	GRANITE results from slow cooling and consists of crystals of quartz, mica, feldspar, and hornblende which are easily visible.
	Large and very small crystals	PORPHYRY is formed from magma that cooled deep in the crust first and then moved nearer the surface. Large crystals are set in a background of very small crystals.
	Very small crystals	BASALT is a dark, dense rock. Crystals are microscopic in size, which tells us that this rock once flowed near or on the surface or the earth.
	Glassy	OBSIDIAN is commonly called volcanic glass. It cooled so quickly, it has no crystals.
	Fragmental	PUMICE is very light, is full of holes, and floats in water. Its origin is volcanic.
Sedimentary Rocks are broken up into small particles by weathering and erosion. These materials, or sediments, are carried by streams and deposited in the oceans in layers. Eventually these layers are buried by more sediments and compressed into rock. Some sedimentary rock is made of tiny particles that settle out of sea water onto the ocean floor, and some is made from the actual remains of plants and animals.	Coarse and fine particles	CONGLOMERATE consists of pebbles and finer material. It resembles concrete.
	Medium coarse particles	SANDSTONE is made of sand particles, principally quartz.
	Fine particles	SHALE, the most common sedimentary rock, is made of visible layers of silt and clay.
	Fine particles	LIMESTONE can be made up of tiny particles of calcium carbonate that settled to the ocean floor, or it can be entirely made of shells or coral.
	Fragmental to compact	PEAT is the compressed remains of plants. Compressed further, it forms lignite, then bituminous coal.
	Crystalline	GYPSUM results from evaporation of water from ancient seas.
Metamorphic Metamorphic means change of form. When rocks are subjected to sufficient heat, pressure, or movement (like faulting), there is a recooking of minerals within the rocks and the minerals are rearranged, in some cases in distinct bands.	Coarse banding	GNEISS was once granite or conglomerate. It has dark and light minerals in wide bands.
	Thin banding	SCHIST may have originally been shale, slate, granite, basalt, sandstone, or limestone. It is usually rich in mica.
	Very fine banding	SLATE, once shale, breaks easily into layers.
	Compact	QUARTZITE was originally sandstone.
	Compact	ANTHRACITE, or hard coal, was originally bituminous coal.
	Crystalline	MARBLE was once limestone.

"The Three Classes of Rocks." Our Wonderful World, An Encyclopedic Anthology for the Entire Family. NewYork:Grolier Incorporated, 1963 (vol. 1, page 10).