The Lava Plain of the North 99 



i.e., the land had been worn down so that the stream currents 

 were sluggish and unable to carry away any but the lighter 

 sediments. There was left spread over the land surface the 

 hard heavy rock materials which the sluggish streams were not 

 able to carry away, such as quartz gravels and gold. 



In early Tertiary (Eocene) time, about 60,000,000 years 

 ago, an upwarping of the land occurred so that the grades of 

 the streams were greatly increased. Most of the fine and light 

 material was carried down and deposited in a body of water 

 that covered the Sacramento Valley, but the coarse and heavy 

 material, such as quartz gravel and gold, accumulated in the old 

 channels. These ancient river gravels furnish the principal 

 placer mines of today. The ancient auriferous gravels of the 

 northern end of the Sierra Nevada were deposited by streams 

 that flowed into the body of water referred to which sur- 

 rounded the northern end of the Sierra Nevada. Some of these 

 gravels, which were once but little above sea level, are now on 

 the summits of the Sierra Nevada at altitudes ranging from 

 5,000 to 7,000 feet above sea level. This indicates that the 

 northern end of the Sierra Nevada range has been uplifted sev- 

 eral thousand feet since these early gravels were deposited. 

 This uplift of the range increased the eroding power of the 

 streams, and as a consequence all the streams flowing down the 

 western slope of the northern portion of the range have cut deep 

 canyons. The canyon of the North Fork of Feather River is 

 nearly 4,000 feet deep. 



A Variety of Lavas Emitted 



Volcanoes of the great lava field have furnished a great 

 variety of igneous rocks. Igneous rocks are distinguished by 

 their structure, the proportions of the mineral silica which they 

 contain, and the minerals of which they are composed. Lavas 

 of different character have been emitted from the same crater 

 at different times. Lavas of the same character have been 

 emitted from different craters. Lava, as it is poured out of vol- 



