38 



PHYSIOGRAPHIC GEOLOGY. 



That, while there are many variations in the courses of the earth's fea- 

 ture-lines, there are two directions of prevalent trends, — the northwes- 

 terly and the northeasterly ; that the Pacific and Atlantic have thereby 

 their positions and forms, the islands of the oceans their systematic 

 groupings, the continents their triangular and rectangular outlines, and 

 the very physiognomy of the globe an accordance with some compre- 

 hensive law. The ocean's islands are no labyrinths; the surface of the 

 sphere is no hap-hazard scattering of valleys and plains ; but even the 

 continents have a common type of structure, and every point and 

 lineament on their surface and over the waters is an ordered part in 

 the grand structure. 



It has been pointed out, first by Professor R. Owen, of Indiana, 1 that the outlines of the 

 continents lie in the direction of great circles of the sphere, which great circles are, in 

 general, tangential to the arctic or antarctic circle. By placing the north pole of a globe 

 at the elevation 23° 28' (equal to the distance of the arctic circle from the pole or the 

 tropical from the equator), then, on revolving the globe eastward or westward, part of 

 these continental outlines, on coming down to the horizon of the globe, will be found to 

 coincide with it; and, on elevating the south pole in the same manner, there will be 

 other coincidences. Other great lines, as part of those of the Pacific, are tangents to 

 the tropical circles instead of the arctic. But there are other equally important lines 

 which accord with neither of these two systems, and a diversity of exceptions when we 

 compare the lines over the surfaces of the continents and oceans. 



Still, the coincidences as regards the continental outlines are so striking that they 

 must be received as a fact, whether we are able or not to find an explanation, or bring 

 them into harmony with other great lines. 



4. SYSTEM IN THE OCEANIC MOVEMENTS AND TEMPERATURE. 



(1.) System of oceanic movements. — The general courses of the 

 ocean's currents are much modified by the forms and positions of the 

 oceans ; but the plan or system for each ocean, north or south of the 

 equator, is the same. This system is illustrated in the annexed figure 

 (Fig. 30), in which all minor movements are avoided in order to pre- 

 sent only the predominant courses. W E is the 

 equator in either ocean ; 30°, 60°, the parallels so 

 named ; N, S, the opposite polar regions : the ar- 

 row-heads show the direction of the movement. 

 The main facts are as follow : — 

 (1.) A flow in either tropic (see figure) from 

 the east, and in the higher temperate latitudes 

 from the west, the one flow turning into the other, 

 making an elliptical movement. The tropical 

 waters may pass into the extratropical regions in 

 all longitudes; but the movement is appreciable 

 only toward the sides of the oceans. 



(2.) A flow of a part of the easterly-flowing 



1 Key to the Geology of the Globe, 8vo, New York, 1857, and Am. Jour. Sci., II. 

 xxv. 130. 





Fig. 30. 





N 



60° 



f.jm" r " r 



%30° 







.30° 



^arn^sm 



A^ 



X * 



