SYSTEM IN THE COURSES OF THE EARTH S EEATURE LINES. 



35 



distinct from an island ? It is a body of land so large as to have the typical 

 basin-like form, — that is, independent mountain chains on either side of a 

 low interior. The mountain borders of the continents vary from 500 to 1.500 

 miles in breadth at the base. Hence a continent cannot be less than a 

 thousand miles (twice five hundred) in width. 



20. 



m. SYSTEM IN THE COURSES OF THE EARTH'S FEATURE LINES. 



The system in the courses of the earth's outlines is exhibited alike over 

 the oceans and continents, and all parts of the earth are thus drawn together 

 into even a closer relation than appears in the principle already explained. 



The principles to which the facts point are as follows : (1) that two 

 great systems of courses or trends prevail over the world, a northwestern and 

 a northeastern, transverse to one another ; (2) that the islands of the oceans, 

 the outlines and reliefs of the continents, and the oceanic basins themselves, 

 alike exemplify these systems ; (3) that the mean or average directions 

 of the two systems of trends are northwest-by-west and northeast-by-north; 

 (4) that there are wide variations from these courses, but according to prin- 

 ciple, and that these variations are often along curving lines ; (5) that, what- 

 ever the variations, when the lines of the two systems meet, they meet 

 nearly at right angles or transversely to one another. 



(1) Islands of the Pacific Ocean. — The lines or ranges of islands over 

 the ocean are as regular and as long as the mountain ranges of the land. To 

 judge correctly of the seeming 

 irregularities, it is necessary to 

 consider that, in chains like the 

 Rocky Mountains, or Andes, or 

 Appalachians, the ridges vary 

 their course many degrees as 

 they continue on, sometimes 

 sweeping around into some new 

 direction, and then returning 

 again more or less nearly to 

 their former course, and that 

 the peaks of a ridge are very 

 far from being in an exact line 

 even over a short course ; again, 

 that several approximately paral- 

 lel courses make up a chain. 



A. Northwesterly system 

 OF TRENDS. — In the southwest- 

 ern Pacific the Neio Hebrides 

 (Fig. 20) show well this linear 

 arrangement ; and even each island is elongated in the same direction with 

 the group. This direction is nearly northwest (N. 40° W.), and the length 





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