TOPOGRAPHY OF THE PACIFIC COAST. 39 



influenced by it in their distribution. Only two of these, the 

 middle and the lower, need here be considered. The middle 

 formation is a tuff which has already been called the Tuscan 

 tuff. Below the Tuscan tuff and above the Cretaceous are grav- 

 els, sands, and clays, which apparently occupy the exact taxo- 

 nomic position of the lone formation of Becker, Lindgren,^ and 

 Turner, and may therefore be appropriately designated by the 

 same name. 



Tuscan tuff. — The Tuscan tuff is composed wholly of volcanic 

 material. It will be considered first, for the reason that it 

 can be most easily identified in different localities, and can be 

 used to great advantage as a reference plane in considering the 

 lone. 



On the western border of the Sacramento valley the most 

 southern exposure yet observed is on Thomes creek, four miles 

 east of Paskenta. From this point it has been traced with vary- 

 ing thickness for fifty miles across all the streams, cutting the 

 eastern margin of the baselevel from Elder creek to Redding. 

 It continues, with interruptions, around the northern end of the 

 Sacramento valley to the thick deposits of similar material in the 

 Lassen Peak region. It thins out to the westward and laps over 

 on the baselevel in such a way as to indicate that the baselevel 

 was formed before the great volcanic eruption which gave birth 

 to the tuff. 



lone formation. — Beneath the Tuscan formation lies the lone, 

 which rests upon the upturned and eroded edges of the Creta 

 ceous (Shasta-Chico) strata with conspicuous unconformity. In 

 the Bald Hills region, northeast of Paskenta, it is composed of 

 clay, and thins out rapidly to the westward against the edge of 

 the baselevel. Farther northward the formation thickens some- 

 what, and contains much gravel, but everywhere it thins out rap- 

 idly to the edge of the baselevel. In the Lassen Peak region, 

 beneath the lava, it has its greatest development, and is many 

 hundreds of feet in thickness. To the northeastward it borders 



^ Geological Atlas of the United States. Text accompanying the Sacramento 

 sheet. 



