JAMES DWIGHT DANA 255 



to have been the first to develop the idea into a definite scientific 

 theory. But the elaboration of the theory into its present form 

 is largely the work of Dana. His earliest discussion of the sub- 

 ject appeared in the Journal of Science in 1847. I n l ater years he 

 returned to the subject again and again; and the theory, as 

 shaped by his maturest thought, appears in the last edition of 

 the Manual. In his earlier writings, his views of the origin of 

 continents and mountains are developed on the assumption 

 of a liquid globe. In later years he abandoned that view, and 

 adjusted his theories to the more probable doctrine of a globe 

 substantially solid. 



Dana's conception of the origin of mountains may be formu- 

 lated somewhat as follows: In the contraction of the earth's 

 interior, the suboceanic crust is the chief seat of subsidence. 

 As the suboceanic crust, in its subsidence, necessarily flattens, 

 so that its profile continually approaches the chord of the arc, it 

 exerts a tangential thrust towards the continental areas. The 

 rather abrupt change in the radius of curvature, in passing from 

 the oceanic to the continental areas, determines lines of weakness 

 along the continental borders, which mark in general the location 

 of the great mountain wrinkles. The crustal wrinkles will involve 

 upward and downward folds geanticlines and geosynclines (in dis- 

 tinction from simple anticlines and synclines, which are foldings 

 of strata on a much smaller scale both in breadth and depth). A 

 gradually subsiding geosynclinal trough along the border of a 

 continent may naturally be kept full of sediment deposited pari 

 passu with the subsidence. Thus strata may accumulate in nar- 

 row tracts to immense thickness, as in the case of the Appalachians, 

 taken by Dana as a type of mountain structure, where the strata 

 are more than six miles in thickness. The progressive subsidence 

 carries the lower strata of the mass to a depth where they are 

 affected by the internal heat of the earth, since the isogeotherms 

 are nearly parallel with the surface. These water-loaded sedi- 

 ments are softened in much greater degree by the high tempera- 

 tures which they encounter than the nearly anhydrous crystalline 

 rocks which they have displaced. The geosynclinal trough at last 



