MOLLUSCA I THEIR SHELLS. 



41 



light. We are looking now at the perpendicular section ; 

 is it not a beautiful object 1 ? 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 editice. These 

 are connected to- 

 gether by an infinite 

 multitude 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 

 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 



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. 



