Ash falling from nearby volcanoes has been known to cover entire forests, and some of these fossil forests have been found with the trees still standing and in an excellent state of preservation. Quicksand and tar are also commonly responsible for the rapid burial of animals. The tar acts as a trap to capture the beasts and as an antiseptic to retard the decomposition of their hard parts. The Rancho La Brea tar pit at Los Angeles, California, is famous for the large number of fossil bones that have been recovered from it. These include such forms as the sabre-tooth cat, giant ground sloths, and other creatures that are now extinct. The remains of certain animals that lived during the Ice Ages have been incorporated into the ice or frozen ground, and some of these frozen remains are famous for their remarkable degree of preservation. MISSING PAGES IN THE RECORD Although untold numbers of organisms have lived on the earth in past ages, only a minute fraction of these have left any record of their existence. Even if the basic requirements of fossilization have been fulfilled, there are still other reasons why some fossils may never be found. For example, large numbers of fossils have been destroyed by erosion or their hard parts have been dissolved by underground waters. Others were entombed in rocks that were later subjected to great physical change, and fossils enclosed in these rocks are usually so damaged as to be unrecognizable. Then, too, many fossiliferous rocks cannot be studied because they are covered by water or great thicknesses of sediments, and still others are situated in places that are geographically inaccessible. These and many other problems confront the paleontologist as he attempts to catalog the plants and animals of the past. The missing pages in the fossil record become more obvious and more numerous in the older rocks of the earth’s crust. This is because the more ancient rocks have had more time to be subjected to physical and chemical change or to be removed by erosion. DIFFERENT KINDS OF FOSSIL PRESERVATION There are many different ways in which plants and animals may become fossilized. The method of preservation is usually dependent upon (1) the original composition of the organism, (2) where it lived, and (3) the forces that affected it after death. Most paleontologists recognize four major types of preservation, each being based upon the composition of the remains or the changes which they have undergone. ORIGINAL SOFT PARTS OF ORGANISMS