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The National Geographic Magazine 



although propagated with different ve- 

 locities, are not always distinguishable. 

 The last are the visual waves, resembling, 

 as Major Dutton says, flat waves on 

 water. They are characteristic of the 

 epicentral tract of many great earth- 

 quakes and are highly destructive. They 

 bear no clear relation to elasticity and re- 

 sult from the passage of the deeper waves 

 from an elastic medium (solid rock) into 

 a feebly elastic medium, such as soil or 

 unconsolidated sediments. They thus 

 account for the ruin often wrought in 

 valleys and in low ground when struc- 

 tures on near-by hills escape. 



EARTHQUAKES VERY FREQUENT IN 

 CALIFORNIA 



The frequency of earthquakes in Cali- 

 fornia is well known, and tremors suffi- 

 cient to rattle the windows of dwellings 

 in San Francisco have in the past been so 

 common as to excite little alarm and 

 arouse but passing interest. The number 

 of quakes recorded in San Francisco 

 from 1850 to 1886 is 254, and 514 addi- 

 tional shocks were noted in the same 

 period in other parts of California. They 

 are undoubtedly more prevalent in the 

 region surrounding the Bay of San Fran- 

 cisco than in the northern or southern 

 extremities of the state. While most of 

 the recorded quakes have effected no 

 damage, others, such as the great shock 

 in 1868, which injured San Francisco, 

 the Owens Valley earthquake in 1872, the 

 Vacaville earthquake in 1892, and the 

 Mare Island earthquake in 1898 were 

 notably destructive. In general, it may 

 be said that the earthquakes in California 

 exhibit the features characteristic of tec- 

 tonic quakes, and the Owens Valley 

 shock is generally ascribed to movement 

 along the great fault limiting the Sierra 

 Nevada on the east. 



A section across central California, we 

 will say from Monterey Bay to Mono 

 Lake — shows three well-marked topo- 

 graphic divisions. On the northeast is 

 the gentle western slope of the Sierra 

 Nevada, about 70 miles broad, which 

 rises gradually from the eastern edge of 



the main interior valley to the crest of the 

 great scarp overlooking the deserts of 

 Nevada. The range is essentially a huge 

 fault block composed of Jurassic and 

 older rocks and partly covered by Ter- 

 tiary lavas. 



The Great Valley is in the main an 

 alluvial plain 50 to 60 miles wide, its 

 northern part drained by the Sacramento 

 River and its southern part by the San 

 Juaquin. Both streams flow into the head 

 of Suisun Bay and their waters find their 

 way across a depression in the third 

 topographic division, the Coast Range, 

 through San Francisco Bay and the 

 Golden Gate into the Pacific. 



The Coast Range separates the Great 

 Valley of California from the Pacific 

 Ocean. It comprises numerous nearly 

 parallel ridges separated by narrow allu- 

 vial valleys and constitutes a generally 

 mountainous belt some 60 miles in width. 

 Both in lithology and structure it pre- 

 sents a marked contrast to the Sierra Ne- 

 vada, although the relations of the two 

 ranges in the northern and southern parts 

 of the state are not as yet fully under- 

 stood. 



THE COAST RANGE IS YOUNG AND IS STIEL 

 GROWING 



The oldest rocks known in the Coast 

 Range are limestones and quartzites,with 

 some crystalline schists, and are exposed 

 at various localities from Point Reyes, 

 north of San Francisco, to San Luis 

 Obispo. These rocks, which are probably 

 Paleozoic, are cut by granite supposed to 

 be of the same general age as the main 

 granitic intrusions of the Sierra Nevada, 

 which are known to be post-Jurassic. All 

 of these rocks, after being above sea-level 

 long enough to be extensively eroded, 

 were submerged and were covered by a 

 series of sediments several thousand feet 

 thick, known, from its characteristic de- 

 velopment at San Francisco and on the 

 north side of the Golden Gate, as the 

 Franciscan, or Golden Gate, series. Al- 

 though the Franciscan consists mainly of 

 sandstone such as forms the well known 

 Telegraph Hill in San Francisco and the 



