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 mme E more perma- 
nent. The tension of the viscòus 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 off Porto 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 
