Mineral-laden waters will drop one kind of substance to take up another which dissolves more readily, and this happens sooner or later when a buried bone or log is encountered. Complications of various sorts enter into the process, but the final outcome frequently is a complete change from one chemical composition to another which is more enduring, the transformation being brought about so gradually and thoroughly that in many fossils the inner structure of the original tissue is as accurately reproduced as the fine detail of surface features. Converted into stone, however, the result is still far from permanent. While yet underground the fossil is subjected to distortion and breakage due to earth movements which bend and dislocate the rock deposits. What causes these upheavals and depressions of the earth’s surface remains the subject of much discussion, but that they have occurred on a large scale and continue to occur is clearly evident. At higher altitudes the surface rocks and fossils are exposed to a larger variety of destructive activities than at lower levels where protective coverings are more likely to be provided and retained. Once stripped of that protection there is little chance for a fossil to survive. Beyond a doubt there are many thousands of tons of prehistoric remains damaged or destroyed each year, by weather and stream erosion. FLORAS AND FAUNAS As the various types of sediments continue to accumulate on land and in water they produce deposits of sandstones, claystones, and limestones which in time may acquire great thickness and cover wide areas of sea floor, or continental surface. Usually there is more or less mixing of sediments resulting in sandy limestones, limy clays, and other combinations. Quite commonly, however, the types remain fairly pure but become arranged in layers which alternate from one kind of material to another. At all times the character of the deposit will depend upon the nature of the rocks which supply the materials, and any fossils that may be produced will consist of such plants and animals as live and die during the time the rock is in the making.