LIFE OF JAMES DWIGHT DANA 



its evolution as an epitome of geological history. Un- 

 doubtedly in this he was right. In the simplicity of its 

 form and structure, and especially in the unity of its de- 

 velopment, 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 develop- 

 ment 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 de- 

 velopment of the continent he worked out in all its details. 

 Whether we accept all these details or not, the idea has 

 become the working theory not only for American geol- 

 ogists, but for geologists everywhere. There can be no 

 doubt that Dana's ideas and Dana's work, especially as 

 systematically embodied in his Manual, constitute a dis- 

 tinct epoch in the history of geological science. 



Nor did he stop with the formal laws of this develop- 

 ment. His active mind could not rest short of inquiries 

 into the causes of these laws ; and for this inquiry his ac- 

 curate knowledge of physics and chemistry admirably 

 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, prob- 

 ably because they were the places of least conductivity 

 and therefore of least transference of heat from within, 

 while contrarily the future ocean basins were determined 

 by the places of greatest conductivity and therefore of 

 most rapid cooling all the way down to the centre, 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 



2^6 



