J. D. Dana on the Origin of Continents. 95 
By reference, therefore, to the principle of unequal contraction, 
and to those subordinate causes of change of level usually ap- 
pealed to by Geologists, ( though treated of commonly as primary 
n order to understand the bearing of the facts, we should 
bring to mind the effects of contraction. 'The more prominent 
are as follows :— 
1. Depressions, provided the contraction be unequal in different 
ts. 
2. Apparent elevations, as a consequence of the depressions ; 
that is, elevations as compared with the lowest level, or with a 
y of water occupying the depressions. 
* We may here mention one or two facts in corroboration of the general theory, 
that the more igneous portions of the globe have contracted most and thereby be- 
bm or example, we the continent of ca reduced toa 
narrow strip of land, just where the great American tract | from east to 
a on of igneous action, not yet entirely extinct; that is, about the 
South America are nearly disjoined by a broad arm of the ocean. This single 
Instance is the only one, through the continent of America, of voleanic eruptions 
east of the great western chain of mountains. : a 
in, the East Indies, another region of perpetual fires, in the earth’s history, 
constitute a cluster of islands separating from Asia the large non-voleanic 
olland, properly a part of a southeastern extension of the continent. Moreover, 
i 0 as 
Jand, without fires, exist in the midst of the group, Borneo being one example, 
equalling in extent half the United States, east of the Mississippi. The Indian 
Ocean, at the same time, bears evidence in its coral islands of a much more ex- 
tensive subsidence. Se 
e this Journal, ii, ii ser., 355. While thus mentioning the name of M. C. Pré- 
vost, we should remember that the theory of contraction, as a cause of the earth’s 
features, dates as far back as Leibnitz, many of whose speculations in science are 
partial application of the principle to the Appalachians. aa : 
e writer does not claim to have presented any new principle, except it may 
be the special cause assigned for the oceanic depressions ; and whether this holds 
true, remains for the future to determine. 
