552 



SCIENCE. 



[N. S. Vol. XX. No. 513. 



proceed with a rapidity which increases 

 with the surface exposed per unit of vol- 

 ume.* Hence these processes lead to mini- 

 mum surfaces. Therefore also the mathe- 

 matics of erosion is essentially identical 

 with that of capillarity. 



Geological climates are as interesting to 

 astrophysicists as to meteorologists and 

 geophysicists. Messrs. Langley and Abbot 

 appear to have evidences of recent varia- 

 tions in solar emanation. If these have 

 been considerable in the course of the 

 period of historical geology, light should 

 be thrown upon them by the paleontology 

 of the tropics. Variations in the composi- 

 tion of the atmosphere must have been very 

 influential in determining both the mean 

 temperature of the earth's surface and the 

 distribution of temperature; but so also is 

 the distribution of water. No theory of 

 the glacial period seems generally accept- 

 ed. Croll's theory is discredited. I have 

 shown to my own satisfaction that the as- 

 tronomical conditions most favorable to 

 glaciation are high obliquity and low ec- 

 centricity of the earth's orbit, f but can 

 not claim any extensive following. If I 

 am right, it should be possible to obtain a 

 definite measiire of geological time in 

 years as soon as the astronomers have com- 

 pleted the theory of secular variations in 

 the planetary system so far as to be able 

 to assign the lapse of time between succes- 

 sive recurrences of low eccentricity and 

 high obliquity. 



A most interesting observation, which 

 promises much light on the past history 

 of the globe, is that lavas and strata in- 

 durated by lavas retain the polarity char- 

 acteristic of the locality in which they 

 cooled.J The time may come when this will 

 lead to determinations of the relative age 



* U. S. Geol. Survey, Mon. XIII., 1888. p. 68. 

 t Amer. Jour. Sci., Vol. 48, 1894, p. O.'i. 

 J Brtinhes and David. Comptes Revdus, Vol. 

 13.3, 1901, p. Ii53. 



of lavas, the duration of periods of erup- 

 tion and possibly even absolute determina- 

 tions of date. 



Geology has long, and with some justice, 

 labored under the reproach of inexactitude. 

 As has been illustrated in the preceding 

 pages, the science is still in the qualitative 

 stage and almost wholly lacks the precision 

 of astronomy. Even its most ardent stu- 

 dents have seldom succeeded in ascertain- 

 ing the quantitative relations between ef- 

 fects and operative causes and have been 

 perforce content to indicate tendencies. 

 Thus geological doctrine is far too much a 

 matter of opinion, but this is hardly the 

 fault of the areal geologist. The country 

 must be mapped both for economic reasons 

 and to accumulate a knowledge of the 

 facts to be explained. Working hypoth- 

 eses the field geologist must have, or he 

 could not prepare his map ; and he is only 

 responsible for living up to the standard 

 of Imowledge of his time. He is continu- 

 ally face to face with phenomena for which 

 physics and chemistry should account, 

 though they have not yet done so, and 

 must accept seeming probabilities where 

 certainty is unattainable. So, too, Kepler's 

 predecessors recorded facts and guessed at 

 generalizations as best they might. 



The physics of extreme conditions still 

 awaits satisfactory exploration. The geolo- 

 gist turns to the physicist for help and in 

 most cases meets with the reply: "We can 

 not tell. Astrophysics is in much the same 

 situation. Astronomers know as little of 

 the distribution of density in the stars or 

 planets as do geologists. Real knowledge 

 of the physics and chemistry of high tem- 

 peratures would be as welcome to them as 

 to us. After all, physical geology is the 

 astrophysics of this, the only accessible 

 planet. Geodesy, too, and terrestrial mag- 

 netism are waiting for the solution of geo- 

 physical problems. How much might be 

 done, Lord Kelvin and Mr. George H. 



