Ransomk.] 



The Great Valley. 



409 



River, and which have already been described. That these heavy 

 local accumulations of sediment can be piled up upon the plain, so 

 as to crowd the San Joaquin River and lakes of the Tulare Valley 

 over to the western side of the Great Valley, is not in accord with 

 the idea that the valley floor is in a state of delicate adjustment, 

 ready to compensate by subsidence for any additional load laid 

 upon it. Had such an equilibrium existed, Tulare Lake could not 

 have been formed by the alluvial accumulation which dams it on 

 the north. On the theory of a sensitive crust, we should expect 

 rather to have the main drainage of the Great Valley closer to its 

 eastern side than to the western ; for on this side has been the 

 heaviest deposition, and, therefore, according to the theory, the 

 greatest subsidence. Whether this would actually be the result 

 brought about under the conditions supposed, is a matter that may 

 be considered open to discussion. The main thing, to which atten- 

 tion is here directed, is the fact that the alluvial fans, the lakes, and 

 the main streams, occupy exactly the relative positions, with regard 

 to each other and to the borders of the valley, that they would 

 have were the floor of the latter regarded as rigid under the load of 

 sediment which has been laid upon it, or subsiding at a rate inde- 

 pendent of that load. 



If, then, the isostasy of the Great Valley be not perfect, there is 

 only one other way in which we can conceive of the adjustment 

 taking place. It is plain that if the theory of isostasy is to be 

 adhered to at all, the idea of a gentle and regular subsidence must 

 be relinquished for a movement that is more or less spasmodic in 

 character. The sediments must accumulate until their weight is 

 great enough to overcome the resistance, and when this happens, 

 there will be a sudden settling down of the loaded area to its new 

 isostatic bearings, to be followed by another period of quiet and 

 preparation. But if this be the nature of isostatic adjustments, then 

 it would seem that the fact, so much insisted upon by the sup- 

 porters of the theory, of the great persistency of extremely shallow- 

 water conditions of deposition from the bottom to the top of many 

 sedimentary series, proves too much for their case. Considering 

 for the present the Great Valley only, the well-borings, as far as the 

 records go. show a rather striking regularity of character in the 



