On some Results of the Earth's Contraction from Cooling. 363 



fectly elastic gases. Hence it is probable that c — c x is greater 

 and c l less for aqueous vapour than would result from the general 

 value of a. 



Athenaeum Club, 

 October 4, 18/3. 



Erratum in No. 306. 



Page 289, last line of text, for p( v i- v i) read^(V x -V). 



XLVI. On some Results of the Earth's Contraction from Cool- 

 ing, including a discussion of the Origin of Mountains. By 

 James D. Dana. 



[Continued from p. 289.] 



V. Formation of the Continental Plateaux and Oceanic De- 

 pressions. 



IN my papers of 1846 and 1847 I attributed the formation 

 of the great plateaux called continents, and of the oceanic 

 depressions, to unequal contraction, observing that the oceanic 

 crust, by being the later in consolidation, became more depressed 

 through the continued contraction than the already firm or less- 

 contracting continental crust. The steps in the process I pro- 

 pose now to consider, as a means of further elucidating the 

 subject. 



1. Unequal contraction a fact. 



The fact of unequal contraction is manifest from the inequa- 

 lity of level that exists over the sphere, dividing it into oceanic 

 depressions and continental plateaux ; and unequal contraction 

 (the material of the crust being essentially the same over the two 

 kinds of regions) implies an unequal rate of cooling. Moreover 

 it is a necessary conclusion that the great areas first consolida- 

 ting should have been first free from that chemical activity and 

 those ebullition-like movements due to escaping vapours which 

 are inseparable from the fluid condition of rocks under merely 

 atmospheric pressure. 



2. Location of the continental areas. 



The areas first to become quiet, and first to cool and consoli- 

 date, would be the shallowest areas — that is, ' those beneath 

 which the solid nucleus of the globe reached nearest to the sur- 

 face ; for this approach to the surface would have been favoured 

 by the chemical quiet, and the less depth would ensure more 

 rapid cooling. 



The solid state of the interior mass, under the Hopkins's 

 theory, is due to the pressure of the outer portion, this pressure 



