In the sedimentary rocks one is apt to find remains of some of the animals and plants that lived at the time the rock was forming. While the soft parts of animals decompose rapidly, shells and bones are likely to be buried in the sediments, and if the conditions have been favorable, these remains may be preserved more or less perfectly. All through the millions of years that sedimentary rocks have been forming in the sea, in lakes, on river flood plains and in wind swept deserts, there was an abundance of life, as much as there is today; and our knowledge of that life is derived from these buried fossil remains, so that fossils have a great historic interest. However as there have lived and died several times as many different kinds of animals as live today, the study of fossils becomes a separate subject, which cannot be treated in this book. Should any collector of rocks and minerals come upon fossils, he is opening a new field, and it will be necessary to turn to other sources for their identification. General books on this subject are scarce, but one or two are given in the literature list. A List of the Elements, the Abbreviations Used for Them, and Their Atomic Weight, Which Is Approximately the Number of Times Heavier They Are Than Hydrogen. Name Oxygen = 16 Aluminium, Al 27 Antimony, Sb 122 Argon, Ar 40 Arsenic, As 75 Barium, Ba 137 Beryllium, Be 9 Bismuth, Bi 209 Boron, B 11 Bromine, Br 80 Cadmium, Cd 112 Cæsium, Cs 132 Calcium, Ca 40 Carbon, C 12 Cerium, Ce 140 Chlorine, Cl 35 Chromium, Cr 52 Cobalt, Co 59 Columbium, Cb 93 Copper, Cu 64 Dysprosium, Dy 162 Erbium, Er 167 Europium, Eu 152 Fluorine, F 19 Gadolinium, Gd 157 Gallium, Ga 70 Germanium, Ge 63 Glucinum, Gl 9 Gold, Au 197