GEOLOGY 



2438 



GEOLOGY 



sphere, and when water began to accumulate 

 on the surface of the earth it gathered in 

 depressions which ultimately became the ocean 

 basins. 



Great Earth Movements. Mountains, pla- 

 teaus and continents have been formed by 

 certain great periodic movements which began 

 in the early stages of geologic history. Nearly 

 all of the mountains are due to the folding, or 

 wrinkling, of the earth's crust, and in a large 

 number of cases the folded mountains are 

 near the borders of continents, as the Coast 

 Ranges of North America. Folded mountains, 

 however, sometimes occur in the interior of 

 continents, as the Urals and the Alps, and the 

 mountains of Central Asia. (The eruption of 

 igneous rocks is responsible for the formation 

 of mountains of the volcanic type, such as 

 Vesuvius and Rainier.) Plateaus are made up 

 of great blocks of strata th,at have been ele- 

 vated by earth movements of even greater 

 magnitude than those which formed the moun- 

 tains. The forces that caused the folded moun- 

 tains took the form of horizontal thrusts; 

 plateaus seem to have been raised by vertical 

 forces. At the surface, the blocks that make 

 up the plateaus are separated by fault-planes 

 (see FAULT). 



The continent-forming movements differ 

 from the plateau-forming movements chiefly 

 in magnitude, the former being much the more 

 massive. Plateaus are simply parts of a con- 

 tinental mass that have suffered additional 

 movement, and they bear about the same 

 relation to a continent that one fault block 

 of a plateau does to the plateau as a whole. It 

 is supposed that plateau- and mountain-form- 

 ing movements began with what geologists call 

 the master movements the sinking of part or 

 all of the ocean bottoms. Simultaneously with 

 the settling of the ocean basins occurred the 

 squeezing up of the continents. Both the con- 

 tinents and the ocean basins are believed to 

 have been formed very early in the history of 

 the earth, and both have been subjected to 

 various changes since then. The basins have 

 suffered further sinking and the land masses 

 further elevation; the tendency, too, has been 

 for the basins to be extended and the conti- 

 nents to be restricted. These movements in 

 turn have been offset by the building up of 

 coastal plains along the borders of continents 

 (see COASTAL PLAIN), and by the depositing of 

 sediment on the ocean floor. It must be re- 

 membered that all of these changes are pro- 

 ceeding at an exceedingly slow rate. E.S. 



Divisions of Time 



Principles of Division. Just as we divide 

 present time into centuries, years and months, 

 so do geologists divide the ages of the earth 

 into longer and shorter periods. They are not 

 all agreed upon some of these divisions, and 

 there is a slight difference in the systems em- 

 ployed in Europe and in America. The divi- 

 sions are founded upon some leading charac- 

 teristic which indicates a great change from the 

 conditions of a preceding time. No attempt 

 is made to measure the length of these divi- 

 sions in years, but the relative length of each 

 period is estimated by the number and thick- 

 ness of the rock systems it includes. A divi- 

 sion including rocks of great thickness is con- 

 sidered to be longer than one whose forma- 

 tions are not so thick. All geologists consider 

 the earth to have been in existence millions 

 of years. 



Eras and Periods. The large divisions of 

 time are called eras, and each era is divided 

 into periods. The names of these divisions in- 

 dicate some 'prominent characteristics, or their 

 order of occurrence. The Archeozoic Era is 

 the era of ancient life; the Proterozoic Era, 



that of earlier life. Paleozoic, the name of the 

 third era, means old life. Mesozoic, the name 

 of the fourth era, means middle life, and Ceno- 

 zoic, the name of the last, means recent life. 

 In the publications of the United States Geo- 

 logic Survey, the term Algonkian is used as a 

 synonym for Proterozoic. The table on page 

 2439 includes the divisions of geologic time 

 generally accepted by American geologists. It 

 should be read from the bottom upward. 



Eras Described. The rock formations of the 

 Archeozoic Era are known as Archean. This 

 system extends downwards to unknown depths 

 and is composed largely of rocks of metamor- 

 phosed igneous origin, though there are some 

 metamorphosed sediments. Its upper limit is 

 generally assumed by geologists to be the base 

 of the oldest system which is dominantly sedi- 

 mentary. The Archean system contains some 

 iron ore and ores of other metals, particularly 

 gold, but these are deposited in comparatively 

 limited amounts. No fossils of any kind have 

 been found in the Archean series; thus there 

 is no clue to the character of the life that 

 then existed. The presence of life during this 



