THE DRIFT-DEPOSITS OF THE NORTHWEST. 291 



under the difficult task of accounting for such change of climate on 

 philosophical principles. It is here that the community of the sciences 

 is beautifully illustrated. Astronomy comes to the aid of her younger 

 sister, and Geology receives from her the solution she could not her- 

 self compass. 



Astronomy makes known three great irregularities in the motions 

 of the earth, requiring thousands of years each for their recurrence. 

 They are 



1. The "precession of the equinoxes," combined with the revolu- 

 tion of the line of the apsides, produces a progression in longitude, of 

 the aphelion place of the earth, bringing about a coincidence of the 

 winter-solstice with the aphelion once in 21,356 years. At the present 

 time our aphelion position occurs in the summer season. 



2. Variation of the inclination of the earth's axis to the plane of 

 its orbit. This passes through a double oscillation in about ten thou- 

 sand years. Its effect is to carry the solstitial point through a small 

 variation in latitude, and to that extent prolonging or withdrawing the 

 influence of the sun's rays in the polar regions. 



3. Change in the eccentricity of the earth's orbit. This irregu- 

 larity passes from maximum to maximum once in about one hundred 

 thousand years. Its effect is to lengthen those seasons nearest the 

 earth's aphelion, and to shorten those nearest its perihelion. At the 

 present time, our aphelion, occurring in the summer season, lengthens 

 the warm months to that amount that, combined with the shortening 

 effect of perihelion in winter, makes a difference of about eight days 

 between the summer and winter months of the year. 



The conditions favorable for polar glaciation are as follows : 



1. Winter-solstice in aphelion. 



2. Obliquity of the ecliptic at rnininmm. 



3. Eccentricity of orbit in maximum. 



The coincidence of these three causes would produce the greatest 

 glaciation. The least multiple of their periods of recurrence is about 

 forty-two million years. The second cause may combine a great many 

 times with either of the others within that interval. Its effect being 

 quite inferior to that of either of the other two, it may be disregarded, 

 and the time required for the combination of the other two would 

 then be 4,200,000 years, which would necessarily be preceded and fol- 

 lowed by a number of approximations. These would occasion corre- 

 sponding periods of increase and diminution in the degree of cold that 

 would not reach the maximum cold incident to their coincidence. 

 Probably the most powerful of the causes enumerated is the occur- 

 rence of the winter-solstice in aphelion, which alone may have pro- 

 duced the last glacial epoch. In that case changes in the prevalence 

 of the ice would be due to the operation of cause No. 2, or the ob- 

 liquity of the ecliptic. The earth is now 622 years past its period of 

 south-polar glaciation in the operation of cause No. 1, and is entering 



