1883,] 453 [Crosby. 



is a principle generally accepted by geologists that the accumu- 

 lation of sediments on the subsiding sea-floors along the margins 

 of continents is attended by an elevation of the isogeotherms and 

 a consequent softening of the crust. Thus are developed the 

 weak zones which yield and are mashed up to mountain-ranges 

 when the crust can no longer resist the growing tangential strain. 

 And when this occurs the distortion or warping of the crust is 

 lessened which means a diminution of the mean height of the 

 continents ; though their margins are elevated by the mountain- 

 making. These periods of catastrophe and mountain-making are 

 regarded as favorable to more or less extensive interchanges of 

 land and sea, of areas of elevation and depression.; for this theory 

 does not require that the continents should be fixed. 



2. The second theory is that developed during the last third of 

 a century, chiefly by Professor Dana, and commonly known as 

 Professor Dana's theory. The main points in this theory, as 

 gathered from the latest expression of Professor Dana's views, 

 are the following : * — The earth, superficially at least, is, and was 

 originally, before it had a solid crust, of unlike composition on 

 different sides. This heterogeneity caused a corresponding dif- 

 ference in heat conductivity. The more rapidly conducting areas 

 cooled fastest and were the first to become covered with a solid 

 crust. Solidification is attended by contraction ; and therefore 

 the newly formed crust must have been heavier than the liquid 

 immediately beneath it. As a consequence it broke up and sank 

 until it reached a liquid stratum of the same specific gravity as 

 itself ; and afterwards the process of crusting and sinking went 

 on until a solid crust was built up from this point to the surface. 

 Through the continued escape of heat this primitive crust is 

 thickened and is still thickening by additions to its lower surface. 

 These first formed portions of the crust became, and will always 

 continue to be, the continents. The remainder of the earth's 

 surface was still liquid, after the solidification of the continental 

 areas was well advanced ; and, of course, as long as it continued 

 liquid its surface was level with that of the crust areas. Finally 

 it became the theatre of a similar process of crusting and sinking, 

 and at last permanently froze over. Now the main point is that 

 the contraction of this inter-continental crust during its formation 

 1 Amer. Jour. Sci. (3), vols, v and vi. 



