AMERICAN 
JOURNAL OF SCIENCE AND ARTS, 
[THIRD SERIES] 
ART. XVIIL.—On some results of the Earth's Contraction from 
cooling ; by James D. Dana. Part V, Formation of the 
Continental plateaus and Oceanic depressions. 
V. FoRMATION OF THE CONTINENTAL PLATEAUS AND 
CEANIC DEPRESSIONS. 
In my papers of 1846 and 1847, I attributed the formation 
of the great plateaus, called continents, and of the oceanic 
depressions, to unequal contraction, observing that the oceanic 
crust, by being the later in consolidation, became more de- 
=— through the continued contraction than the already 
m or less-contracting continental crust. The steps in the 
process I propose now to consider, as a means of further eluci- 
dating the subject. 
1. Unequal contraction a fact. | 
_ The fact of unequal contraction is manifest from the inequal- 
; : 5 
€ material of the crust being essentially the same over the 
Mo, kinds of regions, implies an unequal rate of cooling. 
activity, and those ebullition-like movements due to escaping 
vapors, which are inseparable from the fluid condition of rocks 
2. Location of the continental areas. 
_, The areas first to become quiet, and first to cool and consoli- 
date, would have been the shallowest areas, that is, those be- 
Mm. Jour, So1r—Tarrp es Vou. VI, No. 33.—Sept., 1873. 
