238 Mr J. D. Dana on the Origin of Continents. 



The effects of lateral pressure might in many parts be 

 local or of very limited extent. A contracting area might be 

 made up of several separate areas of contraction not acting 

 together upon any particular line. Even supposing a whole 

 quarter of our globe to exert laterally all the force possible, 

 by a uniform contraction continued till the surface was de- 

 pressed eight miles in depth, the whole effect would be equi- 

 valent to a lateral dislocation of only twelve miles. And in 

 this calculation, we make no allowance for upliftings over 

 the contracting area, which would diminish the action ; nor 

 for a diminution of breadth in the surface of the area, which 

 diminution must be going on if the surface is losing heat. 

 In the remarks w^hich follow relating to this point, America, 

 therefore, is not instanced as an example of what mnst every- 

 where have happened, but of what has here happened. 



The foregoing are the obvious effects of contraction. A 

 Prince Rupert's drop (a drop of unannealed glass) may be 

 referred to for farther illustration. The exterior, owing to 

 its cooling first, is under strong tension, and each particle 

 (or section) in the surface, presses laterally upon its neigh- 

 bour like a stone of an arch upon the one adjoining ; and 

 hence the effect of a simple scratch in causing it to break to 

 pieces, explosively. The earth, had it cooled uniformly over 

 the whole exterior (and were it made of a uniform homoge- 

 neous material), would have been in the same circumstances, 

 the whole crust being under immense tension, yet every- 

 where balanced, and therefore not apparent ; but cooling un- 

 equally, the same actual amount of force has been exerted, 

 yet at different periods, producing, in different parts and in 

 different periods, fractures, depressions, and upliftings. 



We comprehend the effects described more clearly if we 

 remember, as we ought, the common statement, that the 

 highest mountains of the earth are about equal in compara- 



the lateral pressure, pressure from above is essential to this result. But since 

 the soft strata are inelastic, and moreover, in themselves are of vast weight, vfe 

 may conclude that there is sufficient vertical pressure independent of any 

 foreign source. A small hand model appears to be as suggestive of error in this 

 case, as a child's model of a bridge to the inexperienced bridge builder. 



