the Earth's Contraction from Cooling. 43 



both the continental and oceanic areas, and to the fact that the 

 latter are the regions of greatest contraction and subsidence, and 

 that their sides pushed, like the ends of an arch, against the 

 borders of the continents, therefore, along these borders, within 

 300 to 1000 miles of the coast, a continent experienced its pro- 

 foundest oscillations of level, had accumulated its thickest depo- 

 sits of rocks, underwent the most numerous uplifts, fractures, 

 and plications, had raised its highest and longest mountain- 

 chains, and became the scene of the most extensive metamorphic 

 operations, and the most abundant outflows of liquid rock. 



And (b) since the most numerous and closest plications, the 

 greatest ranges of volcanoes, the largest regions of igneous erup- 

 tion and metamorphic action exist on the oceanic slope of the 

 border mountain-chains instead of the continental, therefore the 

 lateral pressure acted most effectively in a direction from the 

 ocean. 



(c) Since these border features are vastly grander along that 

 border of a continent which faces the largest ocean, therefore 

 the lateral pressure against the sides of a continent was most 

 effective on the border of the largest oceanic basin, and for the 

 two (the Pacific and Atlantic) was approximately proportioned 

 to the extent of the basins, — this being due to the fact that the 

 oceanic were the subsiding areas (that is, those which contracted 

 most),' and that the larger area became the most depressed. 



5. The oscillations of level that have taken place over the in- 

 terior of North America, through the geological ages, have in 

 some degree conformed in direction of axis to those of the border 

 regions, all being parts fundamentally of two systems of move- 

 ments — one dominantly in a direction northwestward or from 

 the Atlantic, the other northeastward or from the Pacific. 



6. Owing to the approximate uniformity of direction in the 

 lateral thrust under these two systems through the successive 

 ages (a consequence of the isolated position of the continent be- 

 tween two oceanic basins, transverse to one another in axial 

 direction), mountains of different ages on the same border, or 

 part of a border, have approximately the same trend, and those 

 of the same age on the opposite border (Pacific and Atlantic) 

 have in general a different and nearly transverse trend. Hence 

 "one dial plate for the mountains of the world, such as Elie de 

 Beaumont deduced mainly from European geology, will not 



tension, horizontal force, force acting tangentially, as synonyms. " Lateral 

 pressure " was the term oftenest employed, and it was explained by refer- 

 ence to a Prince Rupert's drop. (See Amer. Journ. Sci. 2nd ser. vol. iii. 

 p. 96 &c.) The action appealed to was not in any way different from the 

 "tangential thrust" of Mallet. 



