Geodes
Geodes are nodules, which, when broken open, are found to be hollow and the cavity lined with one or more minerals. They represent a special case of minerals in a cave. There was in the first place a cavity in the surrounding rock, usually of sand or clay. As the water leached through the surrounding rock, it became saturated with one or more minerals and then coming into the cavity, deposited the minerals, either as crystals, or as a non-crystalline mass, lining the cavity. Thus the inside is often a beautiful cluster of bristling crystals, or it may be simply layer on layer of chalcedony of any color. Before this process had gone so far as to completely fill the cavity, erosion had dislodged the mass, and it has been found. One usually recognizes that it is a geode by the fact that it is far too light to be a solid rock, and then it may be carefully broken. They are characteristic of certain formations; so that having accidentally broken the first one, others can be carefully opened to display the beauty of the interior. The geode illustrated on [Plate 70] is lined with quartz crystals, but near by were found many others, some of which had chalcedony and some jasper as a lining. Such crystallined nodules are usually called geodes so long as they occur in a softer matrix so that they are easily dislodged, and until they reach a size of three or four feet in diameter.