179 



THE CAUSES WHICH 



graduated recurrence o£ warm and glacial periods of which we 

 now enjoy the transition from a more genial. The precession of 

 the equinox, or apparent change in position the constellations of 

 the zodiac have assumed in historic time, interpreted as continual 

 variation in the obliquity of the earth to the sun (as though its 

 axis of diurnal revolution insensibly moved from a to a'), has 

 been often advanced as ruling these geological phenomena. In 

 this way the sun would alternately concentrate its heating rays 

 {a) on the atmosphere nearer the Arctic circles during the summer 

 solstices (a, c), and after a time recede to the region of the 



equator, inducing glacial phenomena at the poles now exposed to 

 the continuous action of cold interstellar space, with a col- 

 lapsing in the earth^s superficies, giving birth to the ensuing 

 wrinkling marked by earthquakes, volcanic action, and land 

 depression, or vice versa f^ 



And that some such law has guided the coeval deposition of 

 strata, I think we may infer from the methodical sculpturing the 

 great continents have undergone from successive formative elevation 

 and depression, evinced by a fringe of submerged plateaux off 

 their coast, abruptly precipitous to 10,000 or 12,000 feet; and 

 also from the methodical way seas have everywhere invaded, and 



* Sir Charles Lyell's view of the thermal distribution of sea and land 

 appears subordinate to this theory ; but the changes in the terrestrial orbit 

 some have advocated do not seem of so much moment if we ask ourselves their 

 import on the present condition of the more remote planets. These cannot be 

 frozen ? 



