Ransome.] 



The Great Valley. 



425 



initial subsidence may have been connected with the general epeiro- 

 genic movements of Pliocene time-;; but if so, it was certainly not 

 due to loading, although it may have allowed the accumulation of 

 a few hundred feet of sediment before the true local subsidence set in. 



Perhaps more conclusive in its testimony is the Wild cat 

 Series * of Pliocene rocks near the mouth of Eel River, composed of 

 soft clays, sandstones, and conglomerates, resting unconformahly 

 upon Mesozoic sandstones. These strata, according to Professor 

 Lawson, were accumulated in a local basin to a thickness of over a 

 mile. Parts of the basement of the series, formerly depressed to that 

 depth below sea-level, now stand about 1,600 feet above it. As the 

 general depression of this portion of the coast in Pliocene times was 

 from i,6do to 1,700 feet (possibly 2,100 feet) lower than at present, 

 the greater part of the subsidence which allowed of the accumula- 

 tion of the Wild cat Series must have been purely local, and the 

 Mesozoic basement, as at Seven Mile Beach, seems to have gone 

 down in obedience to some other force than that due to the weight 

 of the sediments. 



Mountain Building and the Theory of Tsostasy. — A full discus- 

 sion of the relation of isostasy to mountain formation would carry 

 us far beyond the limits of the present paper, but, on the other 

 hand, no treatment of the isostatic theory can be complete without 

 some reference to this special application of it. Of all the facts 

 which have been adduced in behalf of this theory, none, it seems to 

 the writer, afford it so much support as the phenomena of moun- 

 tain origin, as far as we know them, and it may be added that none 

 have been less insisted on. The generalizations of Le Conte, Reade, 

 and others, seem to have shown that the future birth of a mountain 

 range is presaged by the laying down of a great thickness of coarse, 

 off-shore sediments, the deposition being accompanied by a pari 

 passu subsidence. But the significant feature of the process is the 

 apparent fact that the accumulation of such a cylinder lens of sedi- 

 ment can exert so potent an influence upon the earth's crust as to 

 determine along that line the formation of a mountain range. This 

 points to a sympathetic relationship between the movements of the 

 underlying solid crust and the weight of sediments that is not con- 



*Ldwson, toe. tit., p. 255, ct. seq. 



