Ransomk.] 



The Great Valley. 



177 



a steep slope at its edge. They are accordingly of earlier or identi- 

 cal age with it, but not necessarily the latter, as Mr. Diller would 

 have it. There is nothing in the evidence adduced that precludes 

 a Pliocene age for the peneplain, and there are other facts which 

 appear to indicate the latter as the true age.* 



The almost horizontal strata on the west side of the valley, cor- 

 related with the lone formation by Mr. Diller, consist of beds of 

 gravel with some clay, with an observed maximum thickness of 

 64 feet. As the lone formation was not found at the north end of 

 the valley, no direct connection was observed between these beds 

 and the typical lone on the east side, and their reference to that 

 horizon, without more evidence than is given, can be little more 

 than a guess. If, indeed, they be a part of the lone formation, and 

 therefore probably, although not certainly, Miocene, then they are 

 an exception to a general rule of structure early indicated by Whit- 

 ney ; f namely, that whereas on the eastern side of the Great Valley 

 the rocks of Chico (Cretaceous) and later age lie almost undisturbed 

 on the truncated older rocks, with a very slight westerly dip, the 

 rocks of the Coast Ranges, up to and including the Pliocene, have 

 been strongly folded. It must be said in this connection, however' 

 that our knowledge of the geology of the immediate western edge 

 of the Great Valley is as yet very imperfect, and that future investi- 

 gation may reveal Pliocene, or even earlier, beds on the western 

 edge which have escaped the vigorous folding to which their marine 

 equivalents throughout the greater part of the Coast Ranges have 

 been subjected. It is largely in consequence of this general rule of 

 structure that the eastern slope of the Coast Ranges rises with some 

 abruptness from the valley as contrasted with the western slope of 

 the Sierra Nevada. 



Conformably above the lone formation in the northern part of 

 the Sacramento Valley is the Tuscan formation, consisting princi- 

 pally of basaltic tuffs, and varying in thickness from 1,000 feet 

 on the northeast side of the valley, to 50 feet or less along the west- 

 ern border. It is covered in turn by the unconsolidated deposits 



*See Professor Lawson, Bull. Dept. Geol. Univ. of Calif., Vol. r, p. 271, and 

 ante. 



fGeology of California, Vol. I, p. 192. 



