EVOLUTION OF THE EARTH'S OUTLINES. 737 



is intermediate between the two ; and it is interesting to observe 

 that its distance from the small Atlantic is 500 miles, from the great 

 Pacific 2000 ; a ratio of 1 : 4. 



The Azoic nucleus of North America, spreading southward, formed 

 a peninsula in northern New York. Even this bend in the nucleus 

 continues in the finished continent, for New England has the same 

 outline. Its east and south coast-lines are but a repetition of the 

 east and south coast-lines of the old Azoic peninsula. This exact 

 copying of the nucleus by the growing continent proves, better 

 than all other evidence, the grand fact that the progress has been 

 through oscillating forces acting against the stable Azoic nucleus, 

 and also that the system of evolution has been under profound 

 law. 



2. Origin of the Successive Phases in the Earth's Progress. 



1. Epochs. — The epochs in geological history were marked by 

 transitions in the strata and by more or less complete extermina- 

 tions of life. These transitions, as has been already explained (p. 

 398), were directly connected with oscillations in the water-level 

 about the continental areas ; and this water-level was changed by 

 elevations and subsidences either in the continental or the oceanic 

 portion of the crust, or the two united (p. 723). Violent paroxys- 

 mal uplifts and downthrows, and also effusions of heat, were among 

 the events here included, as well as the gentler changes of level 

 that were barely apparent with the passing of a century. But far 

 the larger part even of the seemingly abrupt transitions required 

 only the latter. 



The succession of epochs in history and the development of the 

 earth's features, were, therefore, concomitant results in the same 

 plan of evolution. 



2. Climate. — Three causes have been presented to account for 

 the cooling of the climate of the globe in past time : — 



1. • The decreasing density and cloudiness of the atmosphere, through a 

 diminution of the proportion of carbonic acid and moisture. 



2. The increasing extent and height of the land. 



3. The secular refrigeration of the globe. 



Two other supposed causes are sometimes brought forward : — 



4. A change in the earth's poles. — Such an event would only change 

 the location of the frigid zone or polar climate. When it has been 

 proved that there was a polar climate anywhere in the Palaeozoic 

 ages, and what its location was, it will be soon enough to arrange 

 this among possible causes. Astronomers deny its possibility. 



5. A passing of the earth through warm regions in space during its earlier 



48 



