Geology: The Science of the Earth's Crust. William J. Miller

districts can rarely, if ever, be determined because of the very great length of geologic time and the general slowness of the evolution of organisms. Rocks carrying remarkably similar fossils may really be several thousand years different in age; but this is, indeed, a very small limit of error when one considers the vast antiquity of the earth. Much very accurate and satisfactory work has been done, especially in Europe and North America, in correlating strata and assigning them to their places in the geological time table (see below), but a vast amount of work yet remains to be done before the task is complete.

      Certain types or species of organisms are much more useful than others in the determination of earth chronology. Best of all for world-wide correlations are species which were widely distributed and which persisted for relatively short times. Thus any species which lived in the surface waters of the ocean and was easily distributed over wide areas, while, at the same time, it existed as such only a short time, is the best type of chronologic indicator.

      The known history of the earth has been more or less definitely divided into great eras and lesser periods and epochs, constituting what may be called the geologic time scale. In the accompanying table the era and period names, except those representing earlier time, are mostly world-wide in their usage. Epoch names, being more or less locally applied, are omitted from the table. Very conservative estimates of the length of time represented by the eras and the most characteristic general features of the life of the main divisions are also given.

      PRINCIPAL DIVISIONS OF GEOLOGIC TIME

      (Modified after U. S. Geological Survey.)

Era.	Period.			Characteristic life.	Millions of years estimated
Cenozoic	Quaternary.			“Age of man.” Animals and plants of modern types.	3 to 5.
	Tertiary.			“Age of mammals.” Rise of highest animals except man. Rise and development of highest orders of plants.
Mesozoic	Cretaceous.			“Age of reptiles.” Rise and culmination of huge land reptiles (dinosaurs), of shellfish with complexly partitioned coiled shells (ammonites), and of great flying reptiles. First appearance (in Jurassic) of birds and mammals; of cycads, an order of palm-like angiospermous plants, among which are palms and hardwood trees (in Cretaceous).	5 to 10.
	Jurassic.
	Triassic.
Paleozoic	Permian.			“Age of amphibians.” Dominance of club mosses (lycopods) and plants Primitive flowering plants and earliest cone-bearing trees. Beginnings of back-boned land animals with nautiluslike coiled shells (ammonites) and sharks abundant.	17 to 25.
	Pennsylvanian.
	Mississippian.
	Devonian.			“Age of fishes.” Shellfish (mollusks) also abundant. Rise of amphibians and land plants.
	Silurian.	“Age of Invertebrates.”		Shell-forming sea animals dominant, especially those related to the nautilus (cephalopods). Rise and culmination of the marine animals sometimes known as sea lilies (crinoids) and of giant scorpionlike crustaceans (eurypterids). Rise of fishes and of reef-building corals.
	Ordovician.			Shell-forming sea animals, especially cephalopods and mollusk-like brachiopods, abundant. Culmination of the buglike marine crustaceans known as trilobites.
	Cambrian.			Trilobites and brachiopods most characteristic animals. Seaweeds (algæ) abundant. No trace of land animals found.
Proterozoic	Algonkian.			First life that has left distinct record. Crustaceans, brachiopods, and seaweeds.	25 to 50+
Archeozoic	Archean.			Organic matter in form of graphite (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

      A

      All rocks at and near the surface of the earth crumble or decay. The term “weathering” includes all the processes whereby rocks are broken up, decomposed, or dissolved. A mass of very hard and seemingly indestructible granite, taken from a quarry, will, in a very short time, geologically considered, crumble (Plate 1). During the short span of the ordinary human life weathering effects are generally of very little consequence, but during the long ages of geologic time the various processes of weathering have been slowly and ceaselessly at work upon the outer crust of the earth, and such tremendous quantities of rock material have been broken up that the lands of the earth have everywhere been profoundly affected.

      Most of us have noticed buildings and monuments in which the stones show marked effects of weathering. A good case in point is Westminster Abbey, London, in which many of the stones are badly weathering, some of the more ornamental parts having crumbled beyond recognition since the building was erected in the thirteenth century. In many countries, tombstones and monuments only one or two centuries old are so badly weathered that the inscriptions are scarcely if at all legible.

      What are some of the processes of nature whereby rocks are weathered? In cold countries, and often in mountains of generally mild climate regions, the alternate freezing and thawing of water is a potent agency in breaking up rocks where the soils are thin or absent. On freezing, water expands about one-tenth of its volume and exerts the enormous pressure of over 2,000 pounds per square inch. Nearly all relatively hard rock formations are separated into more or less distinct blocks by natural cracks called “joints” (Plate