92 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 pendent 
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 :aid on thickly in one place favourable for 
its rapid precipitation,—as, for instance, where the water is 
splashed over the surrounding stones and aérated 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 foet 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} 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. 8. 1866, p. 462. 
+ Pengelly, Brit. Assoc. Reports. Kent's Cavern Committee, 10th and 11th 
Report, 1874, 1875. 
