16 PROFESSOR T. MCKENNY HUGHES, M.A. 



in them. When the limestone is weathered away and the 

 iron is oxydised, it colours the earthy residuum red. So 

 cave deposits are often red. When the same process has 

 been carried on at a considerable depth, as, for instance, over 

 the surface of the chalk where covered by the lower Tertiary 

 deposits, the residuum is unoxidised and green.* The 

 carbonate of lime has been carried away in solution, 

 making the spring and river water hard, lining all kettles 

 and boilers with fur. At the mouth of a cave or a 

 spring-head in a limestone district, where the water first 

 gives off part of its carbonic acid, down goes the carbonate 

 of lime which the water can no longer carry, and coats the 

 moss and grass, and anything on which it can collect ; and 

 thus we see in petrifying springs only a proof that the 

 chemical waste, which, under certain conditions, forms caves, 

 is going on continually. 



The quantity of travertine thrown down in some districts is 

 enormous. A great part of Rome is built of this, the Lapis 

 tiburtinus, so named from Tivoli. 



In caves, as the water gets towards the outlet, the car- 

 bonate of lime is precipitated round the edge of a pendant 

 drop or on the margin of some tranquil pool, or, instead of 

 the water eating away the walls of the cave, it coats it over 

 with stalactite, and so protects it from further waste. In 

 doing so it frequently closes up altogether the fissures 

 through which the water once ran. So it grows here, stops 

 growing there is laid on thickly in one place favourable for 

 its rapid precipitation as, for instance, where the water is 

 splashed over the surrounding stones and aerated at a 

 waterfall, while it takes ages to form a thin film in 

 another adjoining chamber. When the great storm of 

 1872 broke up the floors at the mouth of Ingleborough 

 Cave, I saw modern ginger-beer bottles which had been 

 buried a foot deep in the stalagmite. On the other hand, 

 Pengelly records that names cut on the walls of Kent's 

 Cavern as far back as the beginning of the seventeenth 

 century f are only just varnished over, as it were, with a thin 

 stalagmitic coating. From the nature of the case this traverti- 

 nous deposit must be of extremely irregular accumulation, and 

 it is of no value as a measure of the age of the deposits which 

 it covers. On the spray-moistened blades of grass or moss 

 evaporation is rapid, and the travertine soon forms a thick 



* Q. J. G. S. 1866, p. 402. 



t Pengelly, Brit. Assoc. Reports. Kent's Cavern Committee, 10th and llth 

 Report, 1874, 1875. 



