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University of California. 



[Vol. i. 



which are, at least to begin with, independent of loading or unload- 

 ing through transfer of sediment. Having at hand then a general 

 law relating such movements with denudation and sedimentation, 

 which is independent of the theory of isostasy, it remains to be seen 

 how far it is able to account for the facts met with in the Great 

 Valley. If it can do so fully and satisfactorily, then the isostatic 

 hypothesis is, for this particular area, superfluous. 



One of the most common arguments for isostasy is the fre- 

 quently observed fact that sediments laid down in a subsiding area 

 exhibit characteristics which show them to have been successively 

 deposited in very shallow water. The conclusion is drawn that the 

 subsidence must have just kept pace with the sedimentation. But 

 a very little reflexion shows that such a conclusion is not justified, 

 unless it be first established that the sediment was not supplied at 

 a rate more than sufficient to keep up with the sinking. That such 

 an overabundance of sediment has been furnished the subsiding 

 valley through its late geological history, there is excellent reason 

 to suppose. At the present time, such is certainly the case, for the 

 Sacramento and San Joaquin Rivers annually carry large quantities 

 of sediment into San Francisco Bay, where it is partly deposited, 

 and partly swept out to sea by the strong tides setting through the 

 Golden Gate, and dropped upon the bar lying off the entrance to 

 the harbor. The occurrence of coarse gravels and "cobblestones" 

 at a depth of over 2,000 feet, in the very middle of the valley at 

 Stockton, shows that at that period of their history, these streams 

 were vigorous transporters of material, close up to the point at 

 which they issued from the valley (assuming that the outlet of the 

 latter in Pleistocene times was practically where it now is), and 

 must have carried large amounts of finer sediment out to sea as at 

 present. There is accordingly nothing remarkable in the fact that 

 the unconsolidated deposits revealed in the bottom of the Great 

 Valley by borings were all successively deposited very close to 

 sea-level. It i.-> exactly what would be expected in any slowly 

 sinking area which was at the same time supplied with sediment 

 more than enough to make good the depression. 



In strict accordance with the legitimate assumption made at the 

 beginning of this section, it is not necessary to provide an explana- 



