1883.] 461 [Crosby. 



We may safely say that the known facts and the probabilities are 

 all against the supposition that such a difference exists. But 

 without this unproved difference in composition there could be no 

 difference in conductivity and radial contraction and the theory 

 entirely fails. However, granting, for the sake of the argument, 

 the possibility of this diversity of composition ; it still fails as a 

 foundation for the theory, since it could only have been a very 

 transient characteristic of the crust. For, as Professor Dana ad- 

 mits, the elevation or subsidence of large areas of the crust must 

 involve a horizontal displacement of liquid material beneath. 

 Material under the Pacific, for example, being squeezed under the 

 bordering continents. But this process mixes up the matter 

 which by cooling forms continents and ocean-basins through un- 

 equal contraction ; and the areas of high and low conductivity 

 are no longer kept distinct. 



But we may grant farther the possibility of a permanent dif- 

 ference in composition and still doubt the necessity of Professor 

 Dana's inferences. As a rule dense bodies are not only good con- 

 ductors of heat, but they also have low fusing points. This is 

 eminently true of the main constituents of the earth's crust. The 

 most favorable supposition that could be made for Professor 

 Dana's theory would be that the continents have, or had origin- 

 ally, the composition of basalt and the sea- floors the composition 

 of granite ; and in any case the difference in composition must 

 be regarded as similar to, but less rather than greater, than that 

 between basalt and granite. 



But if areas of liquid basalt and granite have the same initial 

 temperature and cool under identical conditions, it does not nec- 

 essarily follow that the basalt areas will solidify first. The ba- 

 salt, on account of its greater conductivity, loses heat more rap- 

 idly than the granite ; and yet, in view of the higher fusing point 

 of the granite, the probabilities are that it would first assume the 

 solid state, the two rocks being wider apart in their fusing points 

 than in their power of conducting heat. This conclusion is 

 abundantly sustained by the observations made on the relative 

 liquidity of basic and acidic lavas. Rhyolite and trachyte solid 

 ify at a high temperature and so rapidly, when exposed to the 

 air, that they are often left in the amorphous or glassy condition 

 of obsidian ; while basalt, congealing at a much lower tempera- 



