298 HOBBS— THE EVOLUTION AND THE [April 24, 



from the relief of strain at the time of the shocks, has not yet been 

 determined. Some writers have dismissed from consideration as 

 " secondary phenomena " most of those visible fractures which 

 first appear at the surface during an earthquake. It seems certain, 

 however, that many of these fractures, at least, as regards both 

 direction and position, are dependent upon the fracture system 

 already present in the underlying rocks; and there is, therefore, need 

 for extended study of the fracture and fault system within the rock 

 basement of each earthquake province. With this study might 

 perhaps be combined the determination of the depth and the earth- 

 quake properties of each of the overlying unconsolidated deposits. 

 Experiments are further necessary in order to determine whether 

 large thicknesses of such deposits are controlled by the same laws as 

 are the thinner ones. 



In every district which has an earthquake history, this record 

 should be examined to learn if possible the points, the lines, or the 

 areas of heaviest shock. Whenever data are sufficiently complete, 

 maps should be compared to represent the approximate distribution 

 of surface intensity for each earthquake, and comparisons instituted. 



Maps of Visible Faults and Fissures and of Block Movements 

 for Special Earthquakes. — It has been pointed out that in the case 

 of a single earthquake only has a map been prepared to show in 

 detail the distribution of the surface faults and the block movements 

 of the ground. Thirty-five years after the event which brought 

 them into existence, these faults have been mapped in detail by Mr. 

 W. D. Johnson, of the United States Geological Survey. It has 

 been possible to prepare maps of portions only of the district 

 affected, and the full results are not yet published. Within the 

 national damain there are at least two other provinces which promise 

 fruitful results from such a study. These are the regions affected 

 by the Sonora earthquake of 1887, and, even more important, the 

 country about Yakutat Bay, Alaska, so profoundly modified in its 

 relief during the earthquakes of 1899. A scientific party with head- 

 quarters upon a surveying vessel, such as we have described, would 

 here find almost unequaled opportunities for securing important 

 data. 



Rate of Mountain or Shore Elevation by Quantitative Methods. 



