Mineralogy and Geology of Nova Scotia. 239 
which they usually occur, we are here presented with cylindrical 
cavities, from half an inch to two inches in diameter, and often 
more than a foot in length. They are mostly vertical or but 
slightly inclined, and sometimes branch in a curious manner. 
The interior of these cylinders is usually coated with a thin 
layer of green earth, over which an incrustation of beautiful 
crystals of heulandite is deposited. A considerable space 
is usually left void in the centre, and the projecting crystals 
are remarkably perfect, exhibiting many curious modifica- 
tions on the primary form. The most common is the re- 
placement of the solid obtuse angles, and the lateral acute 
edges by single planes, thus producing a hexahedral prism with 
dihedral summits. The heulandite is not always crystallized, but 
often entirely fills the tube with lamine, intersecting each other 
in an irregular manner, as if it had attempted crystallization in a 
space too limited to allow room for the crystals to become per- 
fect. They are evidently the product of one crystallization, for 
there are never concentric layers of this mineral in the tubes. 
These cylinders, studded with brilliant crystals of heulandite, 
constitute specimens highly interesting to the mineralogist; but 
the form and position of the cavities may be considered valuable 
evidence in accounting for the originof the trap rocks, Our limits 
will not permit us to dwell on this subject sufficiently to weigh 
the evidence against any theory, but we may venture to hint at 
the evidence which may be derived from their form and position. 
If the cavities were produced by the expansion of an elastic fluid, 
the pressure being equal in all directions, a spherical cavity 
would necessarily be produced; and this might be converted into 
a cylindrical cavity or tube, by the. hardening of that portion 
of the rock to which the upper hemisphere was attached, 
