GEOLOGY. 251 



<leviso(l wliicli is dcpendont, not upon tlie always uncertain data hith- 

 erto ouiploved, but upon observations of intensity wliicli can be ver- 

 itiod by indcpeudeut observers even weeks or months after a great 

 shock, and in which the errors in judgment of the observer have com- 

 paratively little effect upon the result. In this method the various ob- 

 servations upon intensity are first mapped, and a center (which is the 

 epicentrum of the volcanic area) from which the intensities diminish in 

 all directions is found by simple inspection; the intensity observations 

 are then assembled and plotted in the order of their distance from this 

 center upon a diagram in which the ordiuates represent intensities and 

 the abscissa distance from the epicentrum ; when it is found that in all 

 cases there is a point of intiection in tiie intensity curve at a distance 

 from the epicentrum depending upon the depth of the centrum (or focus) 

 which can thus* be determined by a simple formula. This method of 

 determining the depth of earthquake foci is eminently satisfactory, and 

 is unquestionably destined to supplant all others in every case in which 

 it is applicable. By means of it the focus of the Charleston earthquake 

 was shown to be about 12 miles beneath the surface.* (2) Hitherto the 

 determinations of the velocity of earthquake transmission have been so 

 variable that the more cautious seismologists looked upon them with 

 susi)icion, Now, owing partly to the extended use of standard time 

 througliout the United States, the time observations upon the Charles- 

 ton earthquake over the 850,000 square miles affected are satisfactory 

 beyond all precedent; these observations have been carefully reduced 

 and discussed (and in this work Button had the assistance of New- 

 comb), and a coethcieut for the velocity of earthquake transmission, in- 

 comparably more trustworthy than the earlier determinations, has been 

 deduced. The mean result of the various observations gives a velocity 

 of 5,1844-80 meters per second. This coefficient, like the method of de- 

 termining the depth of earthquake foci, is unquestionably destined to 

 supplant all others, and so seismologists are now armed with new means 

 of prosecuting further researches upon the most mysterious of terres- 

 trial phenomena. But Button's contributions are no less important to 

 the geologist than to the seismologist in that they coincide with exi)er- 

 iment and observation upon the elasticity, rigidity, density, and other 

 ])roperties of solid and homogeneous rocks, and so throw light upon the 

 conditi(Hi of the subcrust of the earth far beyond the depths which di- 

 rect examination can ever reach. 



ALTERATION. 



The products of the alteration of rocks by the various agencies to 

 which they are subject are multifarious: The soils are formed by the 

 disintegration and decray of ro(;ky strati; the sediments of seas and 

 lakes are first consolidated into rock, and sometimes subsequently 



Bull, riiil. Soc. Wash., 1888, vol. x, p. 17. 



