

THE EARTH'S FUNDAMENTAL FEATURES. 829 



South, science does not explain. North America began its develop- 

 ment at the north, and in its growth spread southward. The same 

 was true of Europe, although the fact is less pronounced. It may 

 have been so of South America, although her northern extremity is 

 beneath the equator. But of Asia too little is known to warrant the 

 assertion that she has conformed to this principle ; and still less is 

 known of that highest of continents, so far as average level is con- 

 cerned — Africa. Further, astronomy does not allow geology to con- 

 clude that the prevalence of water about the South Pole is sure 

 evidence of less or lower land than at the north, — teaching that if 

 the earth's mass has greater density there it would have more water 

 through its attraction. 



6. Climatal Development. — A globe that has slowly cooled from 

 fusion, has had continents and mountain chains in progress of devel- 

 opment, and has had in the past, as now, a sun that is losing heat like 

 itself, must have been, and be, a globe also of cooling climates. But 

 its orbit has wide variations ; its continents have passed not only from 

 the stage of submergence to that of emergence, but have made the 

 transition back and forth, more or less completely, many times ; and 

 the sun and stars have their varying phases ; moreover, the present 

 era is a time of mild climate compared with that of the Glacial period 

 which preceded it, and hence the cooling of the climates has not been 

 a continuous and regular change, but one by oscillations, in which re- 

 frigeration was real, though often passing through extremes in both 

 directions. Agassiz first suggested that the times of great extermi- 

 nations of life over the globe were epochs of unusual cold, if not of a 

 true glacial era. The first such epoch that is distinctly indicated oc- 

 curred near the close of the Paleozoic era, and it was a time of the 

 most general extermination in geological records. In Britain the Per- 

 mian bears evidence of floating ice in its stony accumulations (p. 431) ; 

 in India the Permian contains bowlders of all sizes, up to five or six 

 feet, and also glacial scratches ; and similar facts have been observed 

 in rocks supposed to be of the same age in the southern part of Africa. 

 The character of the Triassico-Jurassic rocks of Eastern America 

 bear evidence of ice-action in some part of the era of their origin. The 

 exterminations of life at the close of the Cretaceous is attributed to 

 the same cause on page 488 ; but the proof from the condition of the 

 rocks themselves is less decisive. Sufficient, however, is known to 

 establish the fact of oscillation in the development of the earth's 

 climates, and to sustain the view of Agassiz that the cold extremes have 

 been times of unusual catastrophe to the life of the globe. 



Further : changing climates have occasioned changing rates of ero- 

 sion and sedimentary deposition ; have made, over large continental 



