88 Geological Results of the Earth's Contraction. 



Art. XL — A General Revieiv of the Geological Effects of the 

 Earth's Cooling from a state of Igneous Fusion ; by James 

 D. Dana. 



In former papers in this Journal,* the writer has endeavored to 

 illustrate the origin of many of the earth's features, by refer- 

 ence to the necessary consequences of cooling from a state of 

 igneous fusion. In conclusion, a summary of the results arrived 

 at is here offered, in order to aid the reader in a cautious and 

 comprehensive revision of the subject ; for its bearing upon the 

 history of our globe is so important and of so universal a charac- 

 ter, that it cannot receive too close attention. If there has been 

 a state of igneous fluidity, the cause appealed to has acted ; and 

 to reason rightly on many points in geological dynamics, the 

 effects of this prime cause should be first ascertained. Whatever 

 the fact under consideration, be it an elevation, a subsidence, a 

 fracture, earthquakes, igneous ejections, or any of the like opera- 

 tions or their consequences, we cannot be sure of assigning the 

 true explanation, until it is shown whether this grand agen- 

 cy — which commenced with the very beginning of solidification, 

 to end only with cooling itself, — has operated or not in pro- 

 ducing or modifying the result. It is much to be desired that 



mathematical 



t 



The hypothesis of the former fluidity of the earth, we have not 

 deemed it necessary to discuss. The proofs of an approximate 

 uniformity of trend in the earth's features, and consequently of a 

 prevailing structure in the very nature of the crust of our globe, 

 place the question almost, if not quite, beyond doubt. The in- 



Hopl 



i 



that the whole is not now solid, afford still stronger confirmation 

 of the hypothesis, and fully authorize the adoption of it as a basis 

 of reason in 



g- 



* Vol. ii, ii Ser., p. 335, and iii, 94, 176.381,1846, 1847. 

 t In this branch of investigation, principles of the highest 



. -. - question 



of-the interior of the earth and the thickness of the crust. Mr. Hopkins argues 

 that the earth could not have cooled at the surface as long as there was perfect 

 freedom of motion in the igneous fluid, and concludes that "the minimum thick- 

 ness of the crust of the globe, which can be deemed consistent with the observed 

 amount of precession, cannot be less than one-fourth of the earths radius;" also, 

 that the mean inclination of the earth's axis to the place of the ecliptic, can never 

 have changed since solidification commenced. 





