MOLLUSC A: THEIR SHELLS 55 



stage-needle, apply a low power, say 70 diameters, using 

 reflected light. We are looking now at the perpendicular 

 section; is it not a beautiful object? you might fancy your- 

 self looking at one of the 

 noble icebergs that majes- 

 tically navigate the polar 

 seas, when it is rendered 

 porous and laminated by 

 the rains of spring. You 

 see a number of thin hori- 

 zontal tiers or stages, per- 

 fectly parallel and equidis- 



tant, about One-fortieth of Perpendicular; 6, Horizontal section. 



an inch apart, rising above each other like the floors of an 

 edifice. These are connected together by an infinite multi- 

 tude of thin pillars of crystal, or rather leaves, some of 

 which show their edges toward us, others their broader 

 sides, and others are broken off at various distances, the 

 fragments standing up from the floor, or depending from 

 the roof, like stalactites and stalagmites in a cavern. 



This whole series of crystal floors and supporting plates 

 is formed of calcareous matter limestone, in short; but 

 though the latter are set in such close array that the eye 

 cannot penetrate to any appreciable distance between them, 

 their extreme thinness renders the whole structure very 

 light, the interstices being occupied by air. 



But now if I give the stage -needle half a revolution, 

 we shall have the horizontal section presented to the eye. 

 In this aspect we acquire much more information as to the 

 structure. The cut has been made very close to one of 

 the horizontal floors, which we see marked all over with 

 a great number of lines, each of which runs hither and 



