An Introductory Pre-view 5 



south, and a battery of great streams from the east (Feather, 

 Yuba, American, Mokelumne, Tuolumne, Merced, Kings) all 

 pour into the great central basin, whose outlet is via the Golden 

 Gate. But for the "structural valley," known as the Golden 

 Gate, formed by a break or fault in the crust of the earth (a 

 depressed block of the Coast Range) the Great Valley had been 

 a vast lake, and the cities of Sacramento and Stockton, the broad 

 fertile acres to the north and south, had been many feet beneath 

 the water. 



Time is Long in Geology 



To attempt to express in years the time since the period 

 known to geologists as the Jurassic, which marks an important 

 milestone in the geologic history of California, would not mean 

 much to the average mind. As far as it can be estimated it 

 has been 174,000,000 years. This the mind can at best but 

 imperfectly grasp. But something of what has happened can 

 be seen. The work of erosion, which has been going on con- 

 tinuously, marks the great length of time. The tremendous 

 gorges, the yosemites of the western Sierra slope, bear evidence 

 that the time has been long. The formations that lie beneath 

 the surface in the Great Central Valley are made up of rock 

 particles that have been eroded from the high lands and washed 

 by streams and spread out in the great body of water that once 

 covered the area. The vast plain of desert sand that fills the 

 basin that was once a bay or arm of the ocean, and now known 

 as the Colorado Desert, represents the long-time work of a 

 great river which in comparatively recent geologic time has 

 gnawed into the rocks of the Great Plains of Arizona, Nevada, 

 and Utah, and formed the Grand Canyon. 



Geology Offers Key to Best Development of 

 Natural Resources 



These things are all geological in their nature. The study 

 of California becomes the study of geology. Hardly a feature 



