OXY-SALTS AND HALOID SALTS. 53 



of the shells and skeletons of animals, and that it is by 

 the agency of fire chiefly that their forms have been to 

 so great an extent destroyed, the particles that composed 

 them having taken on a crystalline arrangement in the 

 marble. There is one mode of arrangement, differing 

 from the ordinary one, which I will notice in passing. 

 We see it in both marble and common limestone. It 

 gives to marble a grayish color, and an appearance re- 

 sembling the roe of a fish, from the rounded dots with 

 which it is speckled. It is called oolitic marble, and the 

 limestone which is composed of these round grains is 

 called oolite. This term comes from the Greek word 

 don, egg. 



103. Stalactites and Stalagmites. In many limestone 

 caves the carbonate of lime accumulates overhead in 

 shapes like icicles, and these are called stalactites. Ac- 

 cumulations also occur on the floor of the cave similar in 

 form to those which we sometimes see on the ground 

 under dripping icicles, and these are called stalagmites. 

 The resemblance in form in the two cases is very strik- 

 ing. For the chemical explanation of these formations 

 I refer you to 307, Part II. Sometimes the stalactites 

 and stalagmites meet, forming pillars, and these may, in 

 the course of time, become very large, so that, where the 

 roof is high, the appearance by torch-light is magnificent. 

 In TTeyer's Cave, which is a mile and a half in extent, 

 and in some parts forty feet high, many of the stalactites 

 being of a delicate white color, in contrast with the blue 

 limestone of the walls, the scene is exceedingly grand 

 and beautiful. 



104. Magnesian Carbonate of Lime. This mineral, 

 called dolomite, is composed of carbonate of lime and 

 carbonate of magnesia. Its crystals, which are rhombo- 

 hedrons, are often curved, as seen in Fig. 3. Much of 

 the coarser kind of white marble in use for building is 

 dolomite in the granular or imperfectly crystallized 

 form. 



