322 Mineralogy and Geology of apart of Nova S cotia 



roidal shape, in which they usually occur, we are here pre- 

 sented with cylindrical cavities, from half an inch to two in- 

 ches 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 cylin- 

 ders 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 modifications on the primary form. 

 The most common is the replacement 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 laminae, intersecting each other in an irregular man- 

 ner, as if it had attempted crystallization in a space too lim- 

 ited to allow room for the crystals to become perfect. 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, con- 

 stitute specimens highly interesting to the mineralogist ; but 

 the form and position of the cavities may be considered val- 

 uable evidence in accounting for the origin of 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 deri- 

 ved from their form and position. If the cavities were pro- 

 duced by the expansion of an elastic fluid, — the pressure 

 being equal in all directions, a spherical cavity would neces- 

 sarily be produced ; and this might be converted into a cyl- 

 inder, either by the hardening of that portion of the rock to 

 which the upper hemisphere was attached, while by a subsi- 

 dence of the tenacious mass below, containing the other 

 hemisphere, a cylindrical cavity or tube would be produced. 

 The tubes are often bent at right angles, as if the rock had 

 been subjected to an alternate irregular elevation and de- 

 pression. The occurrence of native copper in a similar cav- 

 ity, a few miles to the east of this place, might probably be 

 adduced as evidence that the production of this rock was 

 attended with heat. In the instance referred to, there was 

 a crystal of green analcime attached to a filament of native 

 copper, which, projecting from the rock, probably served it 

 as a nucleus on which to crystallize. The crystals of heu- 

 andite, &c. were doubtless deposited subsequently to the 



