840 /. D. Dana on the Plan of Development 



brought about ? W e detect a plan of progress in the developing 

 germ ; we trace out the spot which is first defined, and thence 

 follow the evolution in different lines to the completed result : 

 may we similarly search out the philosophy of the earth's pro- 

 gress ? 



The organizing agencies in the sphere are — 



1. Chemical combination and crystallization, 



2. Heat, in vaporization, fusion, and expansion, with the cor- 

 relate force of contraction which has been in increasing action 

 from the time the globe began to be a cooling globe. 



3. The external physical agencies, preeminently water and 

 the atmosphere, chiseling and moulding the surface. 



4. The superadded agency of life. 



Of these causes, the first is the molecular power by which the 

 material of the crust has rjeen prepared. The third and fourth 

 have only worked over the exposed surface. But the second, 

 while molecular in origin, is mechanical in action, and in the 

 way of contraction, especially, it has engaged the universal 

 sphere, causing a shrinkage of its vast sides, a heaving and sink- 

 ing in world-wide movements. Its action therefore, has been co- 

 extensive with the earth's surface throughout the earth's history. 

 If a power at all, it has been a dominant power in the great 

 changes, and in connection with the profound structure of the 

 crust received through consolidation, it has wrought out the 

 earth's lineaments, varying them with her years from the first 

 featureless sphere to the bold expressiveness and wrinkles of age. 

 This is the cause that most concerns us at this time. 



There must be system in the intimate structure of the crust. 

 For if it was once fluid, and is now one or two scores of miles 

 thick, all this thickness beyond that of the first film has been 

 produced through gradual, exceedingly gradual and prolonged 

 cooling, adding, by downward increase, to the solid surface 

 arch : and if ice over a pond when thickening in this same 

 way by additions downward to the surface film takes a crystal- 

 line texture perpendicular to this film, as has been proved, we 

 may safely infer that the crystallization of the earth's crust as it 

 slowly thickened would have taken a regular structure, and the 

 more surely since we know that the mineral feldspar, which 

 gives a cleavage structure to granite, is the prevailing mineral in 

 all igneous rocks. Thus we approach some explanation of the 

 prevalence of two great systems of trends in the features of the 

 globe. But this subject we pass by, to the one which more im- 

 mediately concerns us — 'the surface features of the continents. 



The contraction to which I have alluded, going on after a 

 crust was formed over the earth, would necessarily fracture, dis- 

 place, or wrinkle the crust, as the same cause, contraction, wrin- 

 kles a drying apple. The large rind is more than sufficient for 

 the contracted sphere ; and the drawing downward of some parts 



