MOLLUSCA. 



45 



CtTTTLE-SItEI,!.. 



a Perpendicular ; b Horizontal section. 



it floats on tlie surface as buoyantly as if it were ax?tu- 

 ally carved out of cork. 



I cut Avitli a keen knife a little cube out of the 

 '* pounce," and, fixing it on tlie end of the revolving 

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

 i-eflected light. AYe are looking now at the perpei^lic- 

 ular section ; is it not 

 a beautiful object? you 

 might fancy yourself 

 looking at one of the 

 noble icebergs that ma- 

 jestically navigate the 

 polar seas, when it is 

 rendered porous and 

 laminated, by the rains 

 of spring. You see a 

 number of thin horizon- 

 tal tiers or stages, perfectly parallel and equi-distant, 

 about one-fortieth of an inch apart, rising above each 

 other like the floors of an edifice. These are connected 

 together by an infinite multitude of thin pillars of crys- 

 tal, or rather leaves, some of Avhich 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 dis- 

 tance between them, their extreme thinness renders the 

 whole structure very light, the interstices being oc- 

 cupied by air. 



