50 WARREN UPHAMj M.A._, D.SC._, F.G.S.A., ON THE SAN ERANCISCO 



Two Principal Seismic Belts of the Woeld. 



About nine-tenths of all earthquakes occur along the two 

 greatest and longest mountain belts of the world, one mainly 

 encircling the Pacific Ocean, and the other stretching past the 

 ^Mediterranean sea and far east through Asia. 



From Cape Horn northwards through the western hemisphere 

 towers the grand mountain belt of the Andes and the North 

 American Cordilleras, the latter having their newest ranges on 

 their west border, and both adjoining closely the Pacific coast. 

 In Alaska this belt passes westerly, and its outermost south- 

 western range forms the Alaska peninsula and Aleutian Islands. 

 Continuing westerly and in general taking nearly the course of 

 a great circle, the same broad and far prolonged belt of 

 mountain systems comprises Kamtchatka, the Kurile Islands,, 

 Japan, Formosa, the Philippines, Borneo, and Celebes, on the 

 north-western and western side of the Pacific Ocean. In total,, 

 this Pacific coastal orographic belt extends over an arc of about 

 240 degrees, or some 16,000 miles. 



The second great series of mountain chains is a very complex 

 belt passing from east to north-west and west, comprising New 

 Guinea, the Sunda Islands and the Malay peninsula, Anam and 

 .Siam, the colossal Himalayan ranges, the Caucasus, Carpathians,, 

 Balkans, Alps, Apennines, Pyrenees, and Atlas mountains, 

 extending quite across the eastern hemisphere. 



Along these two lines, transverse to each other, one having an 

 extent of two-thirds and the other of half of the earth's circum- 

 ference, the great lateral pressures of the earth's crust, primarily 

 due probably to the cooling and contraction of its interior, have 

 been relieved during the latest geologic ages by plication, faults, 

 and uplifts, producing these most massive and prolonged series 

 of mountains. 



Many earthquakes occur in connection with the eruptions of 

 volcanoes, which are found in many parts of these complex 

 mountainous belts ; but other earthquakes, much exceeding the 

 former in respect to numbers, energy, and immense extent of 

 the areas shaken, are independent of volcanic action, being 

 instead due to fracture and faulting of the rock-formations far 

 from any active or recently extinct volcanoes. Shocks of tlie 

 latter class are called tectonic, meaning that they are associated 

 with processes of mountain-])uil(hng and u})heavalof continents. 

 To this class the shocks (or temblors, if we use the Spanisli 

 word) of San Francisco and Valparaiso belong, and also nearly 



