Ransome.] 



The Great Valley. 



423 



according to Bouguer's formula, which postulates high rigidity, 

 appear to indicate that, as a general thing, gravity is in excess in the 

 low-lying plains, and in defect in the mountainous regions. Many 

 curious and unexplained facts come to light, however. For exam- 

 ple, the boundaries between the regions of excessive and defective 

 gravity do not always correspond with those separating the plains 

 from the mountains. One slope of a mountain chain frequently 

 shows a marked excess of gravity, while, on the other side of the 

 summit, the reverse is the case. Stations situated in level plains, 

 and along valleys, often show conspicuous differences in the force 

 of gravity, this difference frequently being equivalent to the attrac- 

 tion of a plate of rock 60 m. thick for every km. of distance apart. 

 In the flat plains, rivers are separated from each other by regions of, 

 somewhat less excess of gravity. 



These results, as far as they go, are a shade in favor of the the- 

 ory of isostasy, but there are too many unexplained anomalies to 

 render the conclusions very decisive. Von Sterneck himself fre- 

 quently emphasizes their provisional character. For comparison 

 with the work of Putnam, it is interesting to note the general excess 

 of gravity in the low plains of Europe, and the quite conspicuous 

 differences between stations which ought, apparently, to show none. 



Facts from the California Coast Bearing upon Isostasy. — We owe 

 to the work of Prof. A. C. Lawson* the description of three por- 

 tions of the coast of California which are extremely interesting in 

 their bearing on the question of isostasy, viz.: Catalina Island, Seven 

 Mile Beach, and the valley of the Eel River. 



It is evident that if restricted areas of the earth's surface be 

 capable of subsiding independently of any load of sediment that 

 maybe laid upon them, then there should be the possibility of find- 

 ing some such subsiding area, which, owing to some special envi- 

 ronment, has not been made a receptacle for sediment. Such areas 

 might be looked for in the deeper ocean-bottoms, out of reach of 

 ordinary mechanical sediments. That local depressions occur at 

 abyssal depths is a well-known fact and has been emphasized by 

 the recent announcement of the deepest soundings yet made occur- 



* Bulletin of the D^pt. of Geol. of the Univ. of Calif., Vol. 1, pp. 115-160 

 and 241-272. 



