Quartz is the most common mineral deposited in geodes, but calcite, aragonite, dolomite, siderite, pyrite, galena, fluorite, and sphalerite also are found.
Geodes ranging in size from less than one inch to a foot or more in diameter can be gathered from streams where they have accumulated as residual boulders after the rock in which they were enclosed has been eroded.
Hollow geodes are the most desirable because they have better crystals. They can be distinguished from solid ones by their comparative lightness of weight.
Geodes are commonly associated with limestone and dolomite, at some places with shale. In Illinois they can be found most easily in the Warsaw Formation in the area of Nauvoo, Hamilton, and Warsaw, but they also occur in other areas and other formations.
ANIMAL FOSSILS (34)
Prehistoric animals lived in water, on land, and in the air, and left both direct and indirect evidence of their existence, evidence we now call fossils.
Millions of ancient animals died without leaving a trace, but some, especially those that had hard parts such as shells, bones, or teeth, may be found preserved in rocks much as they were when buried beneath sediment on the floor of an ancient sea. Sometimes only imprints of the outside or fillings of the inside of the shells remain, the original material having been completely dissolved. Footprints of land or amphibious animals, burrows made by clams, or holes made by worms also are fossils.
The animals whose remains are fossilized lived and died while the sediments that contained them were being deposited, and they provide clues to the types of life and climate then existing. Fossils of animals characteristic of a certain time are an index to the age of formations where they occur. For example, if a certain trilobite (an ancient relative of the crayfish and lobster) is known to have lived only during a definite time, then all rocks in which it is found are the same age.
Fossils of animals that lived in the sea are exposed in rocks in many parts of Illinois, especially in quarries, river bluffs, and road cuts.