MOLLUSCA : THEIR SHELLS. 



41 



light. We are looking now at the perpendicular section ; 

 is it not a beautiful object? you might fancy yourself 

 looking at one of the noble icebergs that majestically 

 navigate the polar seas, when it is rendered porous and 

 laminated by the rains of spring. You see a number of 

 thin horizontal tiers 

 or stages, perfectly 

 parallel and equi-dis- 

 tant, about one-for- 

 tieth of 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 towards us, others their broader sides, 

 and others are broken off at various distances, the frag- 

 ments 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 



CUTTLE-SHELL. 



a Perpendicular. b Horizontal. 



* In calcareous districts the water trickling through into caverns 

 often forms shapeless masses on the floor or hanging like icicles from 

 the roof ; in the former case they are called stalagmites, in the latter 

 stalactites. 



