122 PHYSIOGRAPHY. [chap. 



water has formed for itself an aqueduct, 240 feet in length, 

 which terminates in a large arch spanning the stream into 

 which the water at one time flowed. All this solid mass 

 must have existed originally in an invisible state of solution 

 in the water of the spring. Such deposits of carbonate 

 of lime are commonly termed travertine, a word supposed 

 to have been derived from the old name. Lapis Tibiirtinus, 

 which was formerly applied to the stone, in consequence of 

 its deposition on a large scale from the calcareous waters of 

 the River Anio at Tivoli, the ancient Tibur, near Rome. 

 At the Falls of the Anio, the travertine has formed bed after 

 bed, to the thickness of four or five hundred feet. 



In consequence of the comparative ease with which 

 limestone yields to the solvent action of water holding 

 carbonic acid gas in solution, this rock is frequently worn 

 by water into holes and caverns. When calcareous water 

 finds its way into the roof of a cav^ern it slowly deposits its 

 burden of carbonate of lime, or at least a portion of it, in 

 a solid form ; and, by long continuance of this action, ulti- 

 mately produces a conical or cyHndrical body hanging like 

 an icicle from the rocky roof Pendent rods of this kiml 

 are termed stalactites} From the point of the stalactite, 

 water slowly drops down upon the floor, and, as this water 

 likewise contains carbonate of lime, another calcareous 

 deposit is formed as a little conical mass seated on the 

 floor; this mass is termed for distinction sake, a stalagmite? 

 As the stalagmite grows in height, it approaches the stalactite 

 above, which continues to grow downwards ; and, ultimately, 

 the two may meet and thus form a solid pillar stretching 

 from floor to roof. Fig. 31 will give some idea of the com- 

 mon shapes assumed by stalactites and stalagmites. It 



1 Stalactite, from <TraKo.<j<ja>, stalasso, to drop. 

 ' Stalagtnite, from (TTaXay/xa,, stalagma, a drop. 



