THE EARTH'S FUNDAMENTAL FEATURES. 827 



dicating that the all-pervading force was in each place at its old work, 

 through all the successive ages, with but small modifications from the 

 changed conditions. 



The force has thus acted as if one in origin and nature, and has 

 manifested at all times the fact that one single system of evolution 

 was in progress. 



The shrinkage in the earth as a whole made its mountain ranges, 

 although these are so localized geographically and chronologically. 

 All the shrinkage that took place during the Paleozoic ages succeeded 

 in producing (as at present understood) in the North American con- 

 tinent only one mountain line ; the Green Mountain part of it after 

 the Lower Silurian era, a small northeastern portion in the Devonian, 

 the Appalachian portion after the Carboniferous. It is not at present 

 easy to calculate the amount of shortening of the circumference of 

 the globe, indicated by the facts ; but, considering the length of Pal- 

 eozoic time, it appears safe to believe that there was fully enough in 

 the sphere between the middle of the Atlantic on the east and the 

 middle of the Pacific on the west, for the result. Europe has little 

 to show for the same era excepting the Urals, on its eastern border. 

 What was done in Asia is yet undetermined. 



4. The System of Trends. — While the relative positions of the 

 continental plateaus and oceanic basins have influenced the general 

 direction of the action of lateral pressure, some unexplained cause — 

 perhaps the existence of a cleavage structure in the crust, or at least 

 the existence of directions of weakest cohesion, in part controlled the 

 courses of fractures and uplifts, somewhat as the warp and woof in a 

 piece of cloth fix the courses of rents, while the direction of the force 

 applied determined the positions and extent of the rents. Force ex- 

 erted at right angles to the lines of structure, and equal along the 

 course, would produce a straight series of rents or uplifts (Figs. 11, 12, 

 p. 19). If not equal along a given line, the series of rents made, 

 taken together, might be oblique or else curving in its axial line (Figs. 

 13, 14, 15). If the pressure were oblique to the structure- courses, 

 the series of rents would be an oblique series, and, as above, either 

 straight or curved. Hence, curves would be necessarily in the system. 



The North Atlantic follows one of the great courses, and the Pa- 

 cific another (page 35). North America is bounded by the two, and 

 hence its triangular form. The coincidence between the trend of the 

 Pacific (northwest and southeast), the mean trend of the Pacific 

 islands (p. 33), and the axis of the coral-island subsidence (p. 583), 

 shows that the ocean in its movements has been one great area of 

 oscillation. The central curving range of that ocean, five thousand 

 five hundred miles long, lies on the southern side of the axis of this 



