LITERARY NOTICES. 



557 



an extreme ellipticity of tbe earth's orbit. 

 The sua is now about three million miles 

 nearer us in the winter than in summer, 

 and the winter (that is, the time from the 

 autumnal to the vernal equinox) is seven 

 days shorter than the summer. In about 

 eleven thousand years from now the con- 

 dition of things will be reversed, and the 

 northern hemisphere will have a summer 

 seven days shorter than the winter, occur- 

 ring while the earth is three million miles 

 nearer its source of heat. About two hun- 

 dred and fifty thousand years ago the eccen- 

 tricity of the earth's orbit was so great that 

 the difference in these seasons was thirty- 

 three days, and the difference between the 

 distance of the earth from the sun at peri- 

 helion and that at aphelion was seven or 

 eight million miles. 



These facts served as the basis for Mr. 

 Croll's theory, who assumed, on the strength 

 of Herschel's authority, that the absolute 

 amount of heat received by the earth during 

 the season which occurred in perihelion was 

 the same as that received during aphelion. 

 He reasoned, therefore, that when the win- 

 ters occurred in aphelion both their in- 

 creased length and the greater distance from 

 the sun would favor the radiation of heat 

 to such an extent that a glacial period would 

 bo produced, especially in those periods when 

 the eccentricity of the earth's orbit was 

 greatest. Dr. Ball comes to the aid of Mr. 

 CroU by showing that the distribution of 

 heat between summer and winter is not in 

 equal quantities, as supposed by Mr. Croll, 

 but that sixty-three per cent of the annual 

 heat received by a hemisphere of the earth 

 falls upon it during the summer — that is, 

 from the vernal to the autumnal equinox — 

 and only thirty-seven per cent during the 

 winter. If, therefore, there was any truth 

 in Croll's original theory. Dr. Ball's dis- 

 covery will greatly increase the efficiency of 

 the cause. 



But the accumulating objections urged 

 by geologists against the theory of Mr. Croll 

 must still apply with all their force. For 

 after Dr. Ball's amendment there is even 

 greater demand than before for geological 

 evidence of a long succession of glacial pe- 

 riods, especially during the later geological 

 eras. But it is the universal opinion of ge- 

 ologists that the Tertiary period was through- 



out one of great mildness of climate, even 

 up to the vicinity of the north pole; yet the 

 Tertiary age doubtless stretched over more 

 than one period of extreme eccentricity of 

 the earth's orbit. Furthermore, the point 

 of glacial radiation in North America is not 

 the north pole, but the region south of 

 Hudson Bay. So clearly is this the case, 

 that President Chamberlin (who has charge 

 of the glacial department of the United 

 States Geological Survey) has adopted the 

 theory that the cause of the glacial phe- 

 nomena of North America was an actual 

 change of the position of the pole ; while 

 others, who can not give their adherence to 

 so improbable a cause, are laying renewed 

 emphasis upon the changes of level in the 

 earth's surface which occurred toward the 

 close of the Tertiary period. 



While, however, we are not convinced 

 of the adequacy of Croll's hypothesis, even 

 as amended by Dr. Ball, we can speak most 

 highly of Dr. Ball's work in bringing clearly 

 before our minds a possible astronomical 

 cause for the Glacial period with which all 

 students of this attractive subject must 

 reckon. The defect in the theory lies not 

 in the mathematical calculations, but in our 

 real lack of knowledge concerning the causes 

 which distribute the heat over the surface 

 of the earth. Meteorology is the science to 

 which we look with most expectancy for 

 further light upon the cause of the Glacial 

 period. The astronomical causes suggested 

 by Dr. Ball's discussion may be so readily 

 masked by slight changes in the direction of 

 oceanic and atmospheric currents produced 

 by relatively slight changes of land level as 

 to be almost entirely ruled out of account. 



Systematic Mineralogy, bared on a Natu- 

 ral Classification. By Thomas Sterrt 

 Hunt, M. A., LL. D. New York : Scien- 

 tific Publishing Company. Pp. xvii + 

 391, octavo. Price, $5. 



This volume aspires to fill a unique place 

 in the literature of mineralogy. As the au- 

 thor notes in his preface, there is no lack of 

 treatises on the science, both determina- 

 tive and descriptive. Still, to a naturalist 

 familiar with the methods of nomenclature 

 in the animal and vegetable kingdoms, the 

 names of mineral species are barbarous, triv- 

 ial, and unmcaninff. This state of affairs 



