184 EARTH FEATURES AND THEIR MEANING 



depressions appear to be the result of local incaving of the cavern 

 roofs. An incaving of this nature may close up an earlier gallery in 

 the cavern and divert the cave waters to a new course. The de- 

 struction of the roofs of caverns through this process of incaving may 

 continue until only relatively small remnants are left. From long 

 subterranean tunnels the caves are thus transformed into subaerial 

 rock bridges that have become known as " natural bridges." The 

 best-known American example is the Natural Bridge near Lex- 

 ington, Virginia. Much grander natural bridges have been formed 

 in sandstone by a totally different process, and must not be con- 

 fused with these limestone remnants of caverns. 



The sinter deposits. Just as water can dissolve the calcare- 

 ous rocks with the formation of caverns, it can under other con- 

 ditions deposit the material which has thus been taken into solu- 

 tion. Its power to hold carbonate of lime in solution is dependent 

 upon the presence of carbonic acid gas within the water. Water 

 charged with gas and dissolved lime carbonate is said to be " hard," 

 and if the gas be driven off by boiling or otherwise, the dissolved 

 lime is thrown out of solution and deposited in a form well known 

 to all housekeepers. 



Hard water flowing in a surface stream, if dashed into spray 

 at a cascade, may deposit its lime carbonate in an ever thickening 

 veneer wherever the spray is dashed about the falls. This material, 

 when cut in section, has waving parallel layers and is known as 

 travertine or calcareous sinter. Some of the most remarkable de- 

 posits of this nature may be seen at the cascade of Tivoli near 

 Rome, and most of the Roman buildings have been constructed 

 from travertine that has been quarried in the vicinity. 



The growth of stalactites. Water, after percolating slowly 

 through the crevices of limestone, where it becomes charged with 

 the carbonic acid gas and with dissolved carbonate of lime, may 

 trickle from the roof of a cavern. Emerging from the narrow 

 crevice, it may give off some of its contained gas and is usually 

 subject to evaporation, with the result that the lime carbonate is 

 left adhering to the rock surface from which evaporation took 

 place. If the water collects upon the cavern roof so slowly that 

 it can entirely evaporate before a drop can form, the entire content 

 of carbonate will be left adhering to the roof. Evaporation is 

 most rapid near the margins and over the center of each drop as it 



