132 THREE CRUISES OF THE " BLAKE." 



If a pressure of two thousand tons to the square foot is suffi- 

 cient to render rock viscous, it would follow that, at a short 

 distance below the bottom of our oceanic basins, the rocks of 

 which are subject to a pressure of nearly seven thousand tons 

 to the square foot, we should soon come to a viscous layer pos- 

 sessing a very high temperature. Now as the upper layer of 

 the rocks of the ocean bottom has a temperature nearly ap- 

 proaching the freezing point, this alone would, according to Mr. 

 Gardner, cause the bottom of the ocean, being under a greater 

 weight than the rocks near the shores, to remain more perma- 

 nent. The tension of the viscous layer, however, would be 

 very great, and would find relief along lines of least pressure, 

 in ridges or along shores of land masses, as is the case with 

 volcanic outbursts or with lines of elevation. For this reason 

 we should naturally seek the lines of greatest elevation in the 

 later geological periods. As is well known, the highest moun- 

 tains are the most recent, and nowhere do we find such great 

 altitudes as in the vicinity of shore lines : as along the west 

 coast of South America, or near the coast of Japan, where the 

 depth is over 4,300 fathoms ; or oft' Poi-to Rico where the eleva- 

 tion above and below the level of the sea is not less than thirty- 

 two thousand feet. 



Many attempts have been made to reconstruct the geography 

 at different geological periods, and to trace the paths of the 

 oceanic currents. These attempts have been more or less suc- 

 cessful, and are probably, in our present state of knowledge, 

 as accurate as the maps of the world made by the ancients 

 compared with the results of modern geodesy. We need not 

 go back to the earlier geological periods, beyond stating in a 

 general way that, as far as we know, the outline only of the 

 continents existed at the time of the silurian, — the skeleton, 

 the framework, as it were, upon which have grown all subse- 

 quent additions. This skeleton allowed a free equatorial cir- 

 culation, broken by larger or smaller islands in the region of 

 the East Indian Archipelago, impeded in a similar way by huge 

 islands on the east and west coasts of equatorial Africa, and by 

 an archipelago occupying the whole northern extremity of South 

 and Central America. Europe was only a series of islands ; the 



