736 DYNAMICAL GEOLOGY. 



The East Indian Archipelago lies between the North Pacific and 

 the Indian Ocean ; and the two, along with the reacting stable con- 

 tinental areas, have ^together modelled out the group. The West 

 Indian Archipelago has a similar position between the North At- 

 lantic and the South Pacific, and hence the resemblances to the 

 East Indian pointed out on page 37. 



The curves along eastern Asia, in the islands and continental 

 mountain-ranges (page 36), seem to show that the tension across 

 the Pacific area, which produced the curves, was unequal along 

 different lines. The courses and positions of the groups of Pacific 

 islands prove that the bottom has its ranges of southeast and 

 northwest elevations and depressions, crossing the ocean ; and this 

 would occasion the unequal tension required. 



Between the directions of the structure-lines and the directions 

 of the acting force, as determined by the oceanic and continental 

 areas, the origin of the prevalent trends and of their frequent 

 curving courses may therefore be explained. 



4. Application to North America. 



The geological progress of North America was an evolution of a 

 continent under the two great systems of forces, the Atlantic and 

 the Pacific. The Appalachians, with their many folds, are a proof 

 of the reality of the former ; and the Rocky Mountains, with their 

 parallel ranges, — the great volcanic chain and Sierra Nevada near 

 the coast, the double crest about the summits, and other ranges 

 across an area one thousand miles in breadth, — are equally a proof 

 of the reality and vast power of the latter. The Azoic land from 

 Lake Superior to the Arctic was the result of an Azoic action of the 

 Pacific force ; and that from Lake Superior to Labrador, of the At- 

 lantic force. This, as shown oh page 136, was the Azoic nucleus, 

 from which the growth went onward, mainly through the oscillating 

 energies to the southeast and southwest. This fixed the position 

 of Hudson's Bay, for it is between the arms of the V. This enlarged 

 the continent to the southeast and southwest, spreading out the 

 strata under the oscillations occasioned, finally doubling the V by 

 adding the Appalachians and the Rocky Mountains, and tripling 

 it later on the west by the Cascade and other ranges. 



Effects of the meeting and crossing of the two forces over North 

 America, that from the southeast and that from the southwest, are 

 seen in the line of the Illinois uplifts and Florida (pp. 320, 531), of 

 the latter system, intersecting the Appalachian chain in eastern Ten- 

 nessee. The area of the uplifted Lower Silurian about Cincinnati 



