236 Mr J. D. Dana on the Origin of Continents, 



though, as we believe with some limitations. They lead us 

 farther to connect the various phenomena, and tell why the 

 ocean and land have their present bounds. 



In order to understand the bearing of the facts, we should 

 bring to mind the effects of contraction. The more promi- 

 nent are as follows : — 



1. Depressions, provided the contraction be unequal in 

 different parts. 



2. Apparent elevations, as a consequence of the depres- 

 sions ; that is, elevations as compared with the lowest level, 

 or with a body of water occupying the depressions. 



3. Fissures. 



4. Ejection of igneous matter, at times, through fissures. 



5. Upheaval along a line of fissure, the surface adjoining 

 being more or less raised. 



6. Upliftings and foldings from lateral pressure. — An arc 

 of the exterior surface being greater than any corresponding 

 arc below the surface, a depression of the hardened exterior, 

 produced by the cooling beneath, would in some instances 

 cause lateral displacements. 



7. An unequal rate of subsidence over given areas in 

 different periods. — Contraction tends to occasion a strain 

 upon the cooled and unyielding exterior, accompanied gener- 

 ally by a consequent diminished rate of subsidence, or a ces- 

 sation of it. This strain increases till it results in fractures ; 

 and following this crisis, subsidence would for a while be 

 more rapid in rate. The strain, or state of tension, might 

 also occasion elevations in some places, within or without 



tioning the name of M. C. Prevost, we should reraemher that the theory of con- 

 traction, as a cause of the earth's features, dates as far back as Leibnitz, many 

 of whose speculations in science are proving to be as well founded as the rigid 

 results of his mathematics. And among the geologists of the present day, De la 

 Beche especially has insisted upon this agency as the general cause of the uneven- 

 ness of the earth's surface, though he stops by stating some of the grand results, 

 without allowing them their full influence as laid down by Prevost. Mr Lyell, 

 in his Travels in N<yrth America, has made a partial application of the principle 

 to the Appalachians. 



The writer does not claim to have presented any new principle, except it may 

 be the special cause assigned for the oceanic depressions; and whether this 

 holds true, remains for the future to determine. 



