EVOLUTION OF THE EARTH'S OUTLINES. 731 



to be a true one. Its sole effect, however, may be to determine the 

 occurrence of earthquakes where another more powerful agency, as 

 that first mentioned, had prepared the conditions and made all 

 ready for the movement. If there are internal tides, and they have 

 this much of power, earthquakes would be most frequent at the 

 semi-annual periods of the highest tides, as Professor Perrey con- 

 cludes to be the fact. 



4. EVOLUTION OF THE EARTH'S GREAT OUTLINES AND RE- 

 LIEFS, AND OF THE SUCCESSIVE PHASES IN ITS 

 PROGRESS. 



1. Evolution of the Earth's Outlines and Reliefs. 



1. General laws as to arrangement. 



In the chapter on the General Features of the Earth (pp. 9-39), 

 it has been shown that there is • system in the arrangement of 

 its reliefs and outlines. "Whatever force, therefore, originated the 

 great mountain-chains originated not merely independent moun- 

 tains, but the system of reliefs of the sphere. 



The general principles connected with this system, announced in 

 the chapter referred to, or brought out in the later pages of the 

 volume, are the following : — 



I. The continents have mountains along their borders, while the 

 interior is relatively low ; and these border mountain-chains often 

 consist of two or three ranges elevated at different epochs. 



II. The highest mountain-border faces the largest ocean, and 

 conversely. 



III. The continents have their volcanoes mainly on their bor- 

 ders, the interior being almost wholly without them, although 

 they were largely covered with salt water from the Azoic age to the 

 Tertiary. Also metamorphic rocks later than the Azoic are most 

 prevalent near the borders. 



IV. Nearly all of the volcanoes of a continent are on that border 

 which faces the largest ocean. 



V. The strata of the continental borders are for the most part 

 plicated on a grand scale, while those of the interior are relatively 

 but little disturbed. 



VI. The successive changes of level on coasts, even from the 

 Azoic age to the Tertiary, have been in general parallel to the 

 border mountain-chains ; as those of the eastern United States, 

 parallel to the Appalachians, and those of the Pacific side, as far 

 as now appears, parallel to the Eocky Mountains. 



