On the Minerals of Trap and the allied Rocks. 53 
specting their origin. According to some writers, they result from 
the process of segregation ;—that is, a separation of part of the 
material of the containing rock during its cooling by the segre- 
gating powers of crystallization; and in illustration of the pro- 
cess we are pointed to the many segregations of feldspar, quartz, 
and mica, in granite and other rocks, the siliceous nodules in 
many sandstones, the pearlstones in trachytes and obsidian. Others 
have thonght them foreign pebbles, enclosed at the time the rock 
was formed. Again, they are described as proceeding from the 
vapors which permeated the rock while still liquid, and which 
condensed as the rock cooled, in cavities produced by the vapors. 
By a few it is urged, admitting that the cavities are inflations by 
vapors like those of common lava, that they may have been filled 
either at the time the rock cooled or at some subsequent time, 
either by crystallization from vapors, or from infiltrating fluids, 
but more generally the latter. 
Of these views we believe the last to accord best with the 
facts. Macculloch in his system of Geology—a work which anti- 
cipated many of the geological principles that have since become 
popular—dwells at length on this subject, and supports the opin- 
ion here adopted with various facts and arguments. Lyell also 
admits the same principles. A review of the facts will enable 
us to judge of its correctness. 
1. In the first place, the cavities occupied by the nodules are 
‘in every respect similar to the common inflations or air bubbles 
in lava. These cavities are open and unoccupied in common 
lava, and may be no less frequently so in the ejections under water ; 
and should they not be expected to fill in some instances by in- 
filtration? They are the very places where an infiltrating fluid 
would deposit its sediment, or collect and crystallize if capable 
of crystallization; and such infiltrating fluids are known to per- 
meate all rocks, even the most solid, and especially if beneath a 
body of water. It is evident, therefore, that we are supporting 
no strange or improbable hypothesis. On some volcanic shores 
one-variety of the process may be seen in action. The cavities 
of a lava may be detected in the process of being filled with lime 
from the sea-water washing over dead shells or coral sand, and at 
times a perfect amygdaloid is formed. But the positions and 
characters of the minerals themselves establish clearly the view 
we support. 
