828 



SCIENCE 



[N. S. Vol. XXXIII. No. 856 



lutely pure, natural races. Wherever nature 

 is experimenting, as discerned by the field 

 zoologist in the observation of geographic 

 series from east to west, and north to south, 

 from humid into arid regions, we are repeat- 

 edly finding geographically continuous series 

 which shade into each other in color, in skull 

 proportion, and limb proportion, and all other 

 characters by continuous degrees of change. 

 Henry Fairfield Osborn 



underground temperatures 

 It is an established fact that as the earth is 

 penetrated below the limit of seasonal changes 

 the temperature is invariably found to rise. 

 Observations made in deep borings, wells, tun- 

 nels and mines have been sufficiently numer- 

 ous over the earth's surface to indicate that 

 the rise of temperature with depth " can not 

 be explained on mere local causes." The rate 

 of temperature increase is not uniform but is 

 found to be quite variable, not only in dif- 

 ferent localities, but frequently in the same 

 boring. This variation of heat increment is 

 doubtless due to a number of causes,' such as 

 differences in the thermal conductivity of 

 rocks which vary in lithologic character, struc- 

 ture and contained water; inequalities of to- 

 pography; circulation of water; chemical ac- 

 tion; compression, etc. Whether the heat 

 increment observed in the superficial zone 

 continues to the center of the earth is not 

 known, as observations are limited to only a 

 little more than 1/4,000 of the earth's radius. 

 Some investigators regard it as more probable 

 that the rise of temperature diminishes below 

 the superficial zone. 



The conducting power of rocks was first ac- 

 curately measured by Forbes,' and later by 

 others. Forbes found that trap rock was the 

 poorest conductor and solid sandstone the best. 

 Sir Archibald Geikie' says, "the lighter and 



^ Chamberlm and Salisbury, ' ' Geology, ' ' Vol. I., 

 1904, pp. 544-547; Geikie, A., "Text -book of 

 Geology," Vol. I., 4th ed., 1903, pp. 63-64. 



' Trans. Boy. Soc. of Ediniurgh, Vol. XVI., p. 

 211. 



= " Text-book of Geology," 4tli ed., 1903, Vol. 

 I., p. 63. 



more porous rocks ofl'er the greatest resistance 

 to the passage of heat, while the more dense 

 and crystalline offer the least resistance." 

 The British Association Committee on Ther- 

 mal Conductivities of Eocks' expressed the 

 resistance of quartz by the number 114, basalt 

 by 273, and cannel coal by 1,538. The same 

 authority" records that heat travels four times 

 as fast in foliated rocks, such as slate and 

 schist, in the direction of cleavage than across 

 it. It has been shown also that thermal re- 

 sistance is lowered by the presence of inter- 

 stitial water. 



The subject of underground temperature 

 attracted attention as early as nearly two cen- 

 turies ago, when observations were made in 

 the mines of Alsace by Gensanne in 1740, who 

 found an increase of 1° F. in 50 feet. Among 

 some of the earlier observers may be men- 

 tioned Saussure, Humboldt, Daubuisson, de 

 Tebra, Forbes and Fox, Henwood, Oordier, 

 De la Hive and Marcet, Phillips and others. 



From 1868 onwards the British Association 

 Reports contain valuable contributions by the 

 committee on underground temperatures, and 

 a summary is published in the volume for the 

 year 1882. In 1886 Professor Prestwich's' 

 valuable contribution on the subject of under- 

 ground temperatures appeared, in which he 

 collated all available data up to that time. 

 This paper was later revised and published in 

 his " Collected Papers on some Controverted 

 Questions of Geology," 1895, pp. 166-279. 

 Observations have been made and data bear- 

 ing on this subject have been contributed at 

 intervals to the literature from 1886 to the 

 present time, with the conclusion that while 

 there is an undoubted increase in temperature 

 downward, the rate is more variable than was 

 at first supposed. 



Professor Prestwich gave the number of 

 different localities and mines where observa- 

 tions were recorded as 248, and the number of 

 stations 530. He found, with but few excep- 



•Eept. Brit. Asso. Adv. of Science, 1875, p. 59. 

 ''Ihid., p. 61. 



» Froc. Boy. Soc. of London, 1886, Vol. XLI., 

 pp. 1-116. 



