I907.] AND MOUNTAIN FORMATION. ^ 387 



to deserve the attention of the careful reader who seeks to follow 

 the historical development of human thought. It is only by a 

 survey of this kind that one gets an adequate perspective of the 

 theories developed in the different ages. Dana's views are given 

 in his " Manual of Geology," 1863, and the most interesting passages 

 are the following : 



(A). On page 29, he points out that the continents are all walled 

 in by mountains erected about their borders, and adds : 



(a) "The continents thus exemplify the law laid down, and not merely 

 as to high borders around a depressed interior — a principle stated by many 

 geographers — but also as to the highest border being on the side of the 

 greatest ocean (first announced in American Jour. Set. (2) XVII, vols. Ill, 

 IV, 1847, and XXII, 335, 1856). The continents then are all built on one 

 model, and in their structure and origin have a relation to the oceans that 

 is of fundamental importance." 



He also observes that the borders of continents are from 500 

 to 1,000 miles wide, and infers that " a continent can not be less than 

 a thousand miles (twice five hundred) in width," otherwise it would 

 not have the characteristic basin form with mountain barriers about 

 a low interior. 



(b) On page 731 he discusses the evolution of the earth's great 

 outline and reliefs, and of the successive phases in its progress, 

 summarizing his conclusions as follows : 



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 con- 

 versely." 



III. " The continents have their volcanoes mainly on their borders, 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 age are most prevalent near the borders." 



IV. " Nearly all of the volcanoes of a continent are on the 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." 



VL " The successive changes of level on coasts, even from the Azoic age 

 to the Tertiary, have been in general parallel to the borders of 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 Rocky 

 Mountains." 



