MEMOIR OF JAMES B. DANA. 469 



present state of geological knowledge, of carrying it out in detail in every 

 part of the earth. He therefore conceived the idea of taking one best 

 known and simplest continent as a type. He regarded the North Ameri- 

 can as such a type-continent and its evolution as an epitome of geological 

 history. Undoubtedly in this he was right. In the simplicity of its form 

 and structure and especially in the unity of its development it certainly 

 deserves to be so regarded. To show this unity of development has been 

 the main object of his geological work. As early as 1856 he compared 

 the evolution of the American continent to the development of an egg. 

 From this point of view (to carry out the idea) the Canadian Archean 

 area may be compared to the germinal disc, about which gathered and 

 organized itself the whole continent. This idea of an organic develop- 

 ment of the continent he worked out in all its details. Whether we ac- 

 cept all these details or not, the idea has become the working theory not 

 only for American geologists, but for geologists everywhere. There can 

 be no doubt that Dana's ideas and Dana's work, especially as systemat- 

 ically embodied in his Manual, constitutes a distinct epoch in the history 

 of geological science. 



Nor did he stop with the formal laws of this development. His active 

 mind could not rest short of inquiries into the causes of these laws ; and 

 for this inquiry his accurate knowledge of physics and chemistry ad- 

 mirably fitted him. A very brief outline of his views may be stated as 

 follows : 



1. In the secular cooling of the earth from primal incandescent liquid 

 condition the continents mark the places of earliest crust-cooling and 

 consolidation — probably because they were the places of least conduc- 

 tivity and therefore of least transference of heat from within — while con- 

 traril)'' the future ocean basins were determined by the places of greatest 

 conductivity and therefore of most rapid cooling all the way down to the 

 center, and therefore also of most rapid radial contraction. But for that 

 very reason the crusting in these places was later, the surface being kept 

 hot by conduction of heat from below. 



2. The more rapid contraction in a radial direction — that is, sinking 

 of the ocean bottoms — not only caused water to accumulate there, but 

 by straightening the curve of the earth-crust pressed against the conti- 

 nents on each side, pushing up their edges and crumpling them into 

 coast ranges, and thus determining the typical form of continents, viz., 

 that of interior continental basins with coast-range rims. He worked 

 out the whole theory of mountain-range formation from this point of 

 view; and if American geologists have been especiall}^ active and suc- 

 cessful in developing the theory of the formation of mountain ranges, it 

 is because Dana led the way. It is easy to see, therefore, why he was so 



LVI— Bull. Geol. Soc. Am., Vol. 7, 1895. 



