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University of California. 



[Vol. i. 



curve produced under conditions of ideal isostasy would be of a 

 somewhat different character from that resulting from tangential 

 pressure — or whatever force that may be which produces such 

 broad, gentle flexures of the earth's crust. But it is doubtful 

 whether we can ever arrive at such criteria, or, if found apply them 

 to an actual geological case. It does appear possible, however, 

 to draw some conclusions from the position of this convex curve, 

 and its relation to the newer formations on the valley's edge. If 

 the Great Valley owes its present shape to isostatic subsidence, 

 then the fact that the great source of the sediment has been the 

 Sierra Nevada, and the probability that during Pleistocene times it 

 has largely been laid down in the form of alluvial fans, would indi- 

 cate that the heaviest deposition, and consequently the greatest 

 subsidence, must have taken place on the eastern side of the valley, 

 close to the foot of the range. The result of this would be a con- 

 tinual loading downward of the lower skirts of the Sierra, in such a 

 way that the newer formations would successively overlap the 

 older, and finally transgress onto the ancient upturned rocks of the 

 " bedrock series " of the Sierra slope, while at the same time the 

 convex curve shown by the river channels would be transferred 

 further and further east. The portion of the Great Valley receiving 

 sediment would steadily increase in area at the expense of the 

 range from which the material was being derived. 



But such is not the observed condition of the sediments on the 

 lower flanks of the Sierra to-day. If we travel from the middle of 

 the valley toward the Sierra Nevada, the Neocene strata soon emerge 

 from the lower alluvium of the main valley, older and older beds 

 coming successively into view. This successive emergence of the 

 older beds is well shown on the Merced River, a tributary of the San 

 Joaquin, draining the famous Yosemite Valley. Near Merced Falls, 

 the base of the Tertiary series (supposedly Tejon-Eocene) rests 

 almost horizontally upon the truncated Jurassic slates at an eleva- 

 tion of about 900 feet, while the younger Tertiary beds occur further 

 west, and at lower elevations. Nor is this elevation recorded by 

 the Tertiary rocks alone; for from Merced Falls westward, the more 

 recent alluvial plain of the river is bordered, particularly on the 

 north, by conspicuous bluffs, which mark the edges of an earlier 



