94 J. D. Dana on the Origin of Continents. 
and beauty of these brick, that they are becoming an article of 
export. Strangers, upon landing here, are surprised that all the 
brick houses are painted of the same color; and their surprise is 
not abated when informed that they are not painted at all—the 
color being that of the bricks themselves.* 
o fossils of any kind have yet been discovered in these lake 
deposits. Cylindrical concretions, of an interesting kind, are 
often found investing the fibres of roots that have penetrated 
small opening through the centre. On breaking across one of 
these concretions, it is seen to have a concentric structure, as if 
made of concentric cylinders. Their form is usually cylindrical, 
tapering at each end. ‘They are much harder than the surround- 
ing mass of clay. 
Arr. XII.—On the Origin of Continents ; by James D. Dana. 
gions of eruption. Hence it was inferred that contraction must 
have taken place to the greatest extent over the parts now oceanic, 
just as any cooling sphere becomes depressed on the side which 
cools last. This was shown to correspond with the actual his- 
tory of our globe, inasmuch as an increasing depth in the ocean 
cavity would necessarily leave more and more land above water 
in successive epochs, as accords with observations. It was ob- 
served that the hypothesis was farther borne out by facts: for 
while it appears that the land has, on the whole, been increasing 
in extent, even through the tertiary era and subsequent to it, the 
ocean’s bottom has actually subsided several thousand feet within 
a late period, as shown by the coral islands scattered over the 
wide Pacific.{ 
large extent, too cold for corals, proves nothing against the hypothesis. On the 
ance of some points of land by submergence. All existing Atlantic islands are of 
igneous origin except the Falklands, to the east of Tierra del Fuego. 
