MINERALOGY. 403 



to build in many branchings until the ridge reaches to 

 the surface of the sea at low water, after which the sea 

 washes upon it fragments of coral, driftwood, sea grass, 

 portions of stranded ships, etc. These substances de- 

 compose and form soil, which gradually accumulates, 

 and, as the sea birds flock there in great numbers, the 

 seeds transported by them from other climes spring up, 

 and aid in forming islands which are at length occupied 

 by animals, with man at their head.* 



Calcareous Tufa or Travertin is a deposit of car- 

 bonate of lime formed by depositions of water from 

 springs containing that substance in solution. The con- 

 cretionary deposits found depending from the roof of 

 caves are called Stalactites, the portion encrusting the 

 floor Stalagmite. The first is formed by the percola- 

 tion of water through limestone rocks, and the calcareous 

 particles which were dissolved and subsequently left on 

 the roof of the cavern by the evaporation of the water. 

 Stalactites assume many grotesque forms, but mostly 

 hang like icicles, and gradually increase by the further 

 deposition of stony particles, in concentric rings. Some- 

 times the stalactite and stalagmite meet, and, joining, 

 form pillars extending from the floor to the roof of the 

 cavern. 



* A cluain of coral islets four hundred and eighty geographical 

 miles in length has long been known as the Maldives. Volcanic 

 agency, also, often lifts the reef far above the waters. Great masses 

 of Guano, often extending many fathoms deep, are found in these 

 homes of the sea birds. A well known fertilizer of soils, guano has 

 now become an article of commerce, and is brought from the coral 

 reefs of the South Sea, where sea birds have congregated unmolested 

 for centuries uncounted. See LTELL'S GEOLOGY, p. 172. 



