30 PHYSIOGRAPHIC GEOLOGY. 



rior. The mountain-borders of the continents vary from 500 to 

 1000 miles. Hence a continent cannot be less than a thousand 

 miles (twice five hundred) in width. 



3. SYSTEM IN THE COURSES OF THE EARTH'S FEATURE- 

 LINES. 



33. 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 established by the facts are as follow : That (1) 

 two great systems of courses or trends prevail over the world, a north- 

 western 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 principle, and that 

 these variations are often along curving lines; (5) that, whatever 

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

 nearly at right angles or transversely to one another. 



34. (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 de- 

 grees 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 parallel courses make up a chain. 



A. Northwesterly system of trends. — In the southwestern Pacific, 

 the New Hebrides (fig. 23) show well this linear arrangement; and 

 even each island is elongated in the same direction with the group. 

 This direction is nearly northwest (1ST. 40° W.), and the length of the 

 chain is 500 miles. New Caledonia, more to the southwest, has 

 approximately the same course, — about northwest. Between New 

 Hebrides and New Caledonia lies another parallel line, the Loyalty 

 Group. The Salomon Islands, farther northwestward, are also a 

 linear group. The chain is mostly a double one, consisting of two 

 parallel ranges, and each island is linear, like the group, and with 

 the same trend. The course is northwest-by-west } the length 

 600 miles. 



