| e {|the nautilus (cephalopods). Rise | Paleozoic | {|and culmination of the marine |17 to 25. |Silurian. o {|animals sometimes known as sea | | f {|lilies (crinoids) and of giant | | {|scorpionlike crustaceans | | I {|(eurypterids). Rise of fishes and | | n {|of reef-building corals. | +-----------n-{+------------------------------------+ | v {|Shell-forming sea animals, | | e {|especially cephalopods and | |Ordovician.r {|mollusk-like brachiopods, abundant. | | t {|Culmination of the buglike marine | | e {|crustaceans known as trilobites. | +-----------b-{+------------------------------------+ | r {|Trilobites and brachiopods most | |Cambrian. a {|characteristic animals. Seaweeds | | t {|(algæ) abundant. No trace of | | e {|land animals found. | -----------+-----------s-{+------------------------------------+--------- | |First life that has left distinct | Proterozoic|Algonkian. |record. Crustaceans, brachiopods, | | |and seaweeds. | -----------+--------------+------------------------------------+ | |Organic matter in form of graphite |25 to 50+ Archeozoic |Archean. |(black lead), but no determinable | | |fossils found. | -----------+--------------+------------------------------------+--------- The length of time represented by human history is very short compared to the vast time of known geological history. The one is measured by thousands of years, while the other must be measured by tens of millions of years. Just as we may roughly divide human history into certain ages according to some notable person, nation, principle, or force as, for example, the "Age of Pericles," the "Roman Period," the "Age of the French Revolution," or the "Age of Electricity," so geologic history may be subdivided according to great predominant physical or organic phenomena, such as "the Appalachian Mountain Revolution" (toward the end of the Paleozoic era), the "Age of Fishes" (Devonian period), or the "Age of Reptiles" (Mesozoic era). In the study of earth history, as in the study of human history, it is important to distinguish between events and records of events. Historical events are continuous, but they are by no means all recorded. Records of events are often interrupted and seemingly sharply separated from each other. CHAPTER II WEATHERING AND EROSION