DYNAMICAL GEOLOGY. 305 



pression to the theory of horizontal compression in explanation 

 of the origin of mountains. The early papers by Dana upon 

 crust-movements were published in the American Journal of 

 Science in the years 1846 and 1847. In them Dana boldly 

 contested the possibility of continents and mountains being 

 raised by the expansive force of subterranean vapours and the 

 ascent of rock-magma; and he also dissented from the 

 gravitation theory of his compatriot, James Hall (1859), 

 according to which the gradual accumulation of sedimentary 

 masses in areas of subsidence must, on account of the altered 

 equilibrium, give rise to folding and fracture of the crust, and 

 consequently to mountain-chains. Hall's idea was to a great 



j extent a modification of previous suggestions by Babbage and 

 Herschel, but these investigators had attributed the subse- 

 quent uplift of thick deposits in areas of subsidence to the 

 expansion of the sediments on account of the high temperature 

 in their deeper horizons. 



In common with Descartes, De la Beche, Cordier, Elie de 

 Beaumont, and others, Dana considered the fundamental 

 cause of crust - deformation to be the slow cooling and 

 contraction of the earth's nucleus. But he made a closer 



I geological investigation than any previous observer of the 

 precise mode of action displayed by the contracting 



! forces. 



Dana assumed that the orographical limits of continents 

 and mountain-chains were determined by certain pre-existing 



| lines of minimum resistance (cleavage-lines) associated with 



I inequalities of thickness and temperature in the earth's crust. 



I He then argued that as the primitive earth cooled, the first 

 crust-blocks that consolidated formed continents, and the 

 pressure caused by shrinkage was most intense at the 

 continental margins. There the greatest mountain-systems 

 developed, and as a rule the height of a marginal mountain 



as well as his comprehensive works on Zoophytes and Crustaceans, are 

 amongst the finest productions in the literature of scientific travel. Dana 

 1 was a Professor at Yale University from 185010 1894, and died on the I4th 

 April 1895. He was distinguished as a zoologist, geologist, and mineral- 

 ogist ; his high merits were recognised in England by the award of the 

 Wollaston and Copley medals. His Text-book of Geology, published in 

 1863, has since passed through several editions, and has had a marked 

 influence on geological thought and progress. Over a hundred papers 

 by Dana have appeared in the American Journal of Science, and they 

 treat almost every subject of general geological interest. 



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