Fe, ON CRYSTALS IN THE CAVITIES OF MINERALS. 
that tint is less brilliant than in the real imbedded crystals. I conceive, there- 
fore, that they are crystallized cavities, having their inner surfaces coated with a 
doubly refracting crust. This is, in itself, a very natural supposition, seeing 
that the fluid may have discharged its gaseous portion, and left behind it the 
matters which it held in solution. The cavities, however, of this kind, which I 
have described in a former paper, have no depolarising action; and I find that 
those now under consideration have regular axes of double refraction. Hence, 
the matter which covers them must be a regular crystalline shell, with optical 
and crystallographic axes—a phenomenon which has no parallel in mineralogy. 
St LEONARD’s COLLEGE, ST ANDREWS, 
February 15. 1845. 
