378 



University of California. 



[Vol. t. 



called by Mr. Diller the Red Bluff formation, which fills up all the 

 lower portions of this part of the valley. 



The great alluvial plain proper of the Sacramento River, which 

 may be said to begin near the town of Red Bluff, widens out rapidly 

 to the south into the characteristic level expanse of the Great Val- 

 ley, and the low rounded foothills recede on either hand. The only 

 interruptions to this nearly level plain, stretching for over 300 miles 

 to the southeast, are the Marysville Buttes, which emerge abruptly 

 from the alluvium to a height of over 2,000 feet above the surface 

 of the valley at about latitude 39 15', thirteen miles northwest of 

 the town of Marysville. As can be seen from the Marysville Folio 

 of the U. S. Geological Survey, they consist of a compact cluster 

 of sharp peaks rising from an almost circular base of about ten 

 miles in diameter. The highest point is 2,128 feet above sea-level. 

 According to the work of Messrs. Liridgren and Turner,* they 

 represent a volcano of rather remarkable type, consisting essen- 

 tially of a core of massive andesite, surrounded by sediments and 

 tuffs dipping away from it on all sides. These sediments have 

 been identified as belonging to the lone (Miocene?) and Tejon 

 (Eocene) formations, which have a wide distribution about the 

 borders of the valley, but have nowhere else been exposed in its 

 middle portions. The age of the volcano was probably late Neo- 

 cene. The base of the Buttes is undoubtedly deeply buried in the 

 alluvium of the valley, from which they rise like islands from the 

 sea. Borings in the tule-land, south of the Buttes, passed for 400 

 feet through unconsolidated deposits without reaching the beds 

 which occur upturned upon their flanks. 



The Sacramento River and its tributary, the Feather River, "pur- 

 sue a winding course on low ridges, "f built up by the rivers them- 

 selves, as is common in streams meandering through a flood-plain. 

 In the case of the Yuba River, which issues from the foot-hills of 

 the Sierra to the east of Marysville, the amount of detritus run into 

 the stream through mining operations has caused it to build up a 

 low sandy alluvial fan upon which it shifts its course each season. 



* Marysville Folio, U. S. Geol. Survey. 



t Ibid. 



