STALACTITES I 3 I 



gradually converted from a carbonate into a phosphate of lime. 

 Such is believed to be the mode of origin of the phosphatic rock of 

 Florida and the West Indies. On the other hand, the phosphatic 

 nodules of South Carolina are regarded as due to the action of 

 swamp water upon underlying shell rocks, though the source of 

 phosphoric acid is not well understood. 



Cave Deposits. — The chemically formed cave deposits are due 

 to the solution and redeposition of carbonate of lime. Caves are 

 very generally found in limestones, and the percolating waters 

 which make their way through the roof of a limestone cavern 

 always have more or less CaC0 3 in solution. A drop of such 

 water, hanging from the cavern roof, will lose some of its CO 2 , 

 upon the presence of which the solubility of the CaC0 3 depends, 

 and deposit a little ring of the lime salt. Successive depositions 

 will lengthen the ring to a tube, and then the tube will be built up 

 by layers on the inner side, until it becomes a cone. At first, the 

 deposit is white, opaque, and very friable, crumbling at a touch, 

 but repeated depositions fill up the interstices of the porous mass 

 and convert it into a hard, translucent stone, which assumes a 

 crystalline structure through the development of calcite or ara- 

 gonite crystals. The masses, thus formed, that depend from the 

 roof of the cavern, are called stalactites. After hanging for a time 

 from the roof, the drop of water falls to the floor of the cave, and 

 there, in similar fashion, deposits a little layer of CaC0 3 , which 

 gradually grows upward into a cone. This is a stalagmite, and 

 differs from the stalactite only in the fact that it grows upward 

 from the floor, instead of downward from the roof. The stalag- 

 mite is, of course, exactly beneath the stalactite, and as long as 

 the water continues to follow the same path, the two cones are 

 steadily, though very slowly, increased both in height and thick- 

 ness, until they meet, unite, and form a pillar extending from floor 

 to roof of the cavern. 



These deposits form the most curious and beautiful features of 

 limestone caverns. The stalactites assume all manner of shapes, 

 determined by the way in which the water trickles over them, and 

 the abundance or scantiness of the water supply. Fantastic and 



