152 EXCA VA TION OF CA VES. 



other parts of the South of England. Where the cliffs waste more 

 rapidly than the tiny streams can cut them away, waterfalls are shot 

 over them into the sea. 



The perennial springs draw their water from the porous beds 

 among the regular geological formations ; and all the tributaries of the 

 Thames are derived from limestones or sandstones, usually at the 

 point where they rest on clays. The fact that the water comes from 

 limestone prepares us to find that it has dissolved from the rock, and 

 holds in suspension in the river a large amount of solid matter. This 

 solid matter, in the waters of the Thames and its tributaries, varies 

 from 20 to 33 grains in the 100,000 grains of water, the average quan- 

 tity being always upwards of 20 grains to the gallon. This invisible 

 matter consists chiefly of carbonate of lime, but in the tributaries 

 one-fifth of the amount, or more, may consist of sulphate of lime. In 

 the lower part of the course of the river the amount of the solid 

 matter is somewhat less, but every day about 1000 tons of carbonate 

 of lime and 238 tons of sulphate of lime are said to be carried to the 

 sea, with smaller quantities of carbonate of magnesia, chlorides of 

 sodium and potassium, sulphates of soda and potash, some silica and 

 a little iron, alumina and phosphates, making in all about 1500 tons 

 delivered into the ocean daily by the Thames. Large as this amount 

 is, the quantity removed is so small that, according to the estimate 

 of Professor Prestwich, the surface denudation of the Thames basin at 

 this rate by chemical means, would amount to less than one-tenth of 

 an inch in a century, so that in the course of 13,200 years about 

 one foot in thickness would be removed from the Chalk and Oolite 

 districts. 



Excavation of Caves. When, however, the water is condensed 

 into a narrow stream, as it often is in flowing under ground, its 

 solvent action is particularly impressive. Charged with carbonic acid 

 from the air, and further enriched with the same substance from 

 decaying vegetation, the surface waters in the mountain -limestone 

 country often disappear in fissures, called "swallow holes." The 

 water here enlarges the fissure by dissolving the limestone, deepens 

 the bed over which it flows, and gradually eats out the lofty chambers 

 known as caverns. Many caverns have a branching ground-plan like 

 a river with its tributaries, showing that when the infiltrated waters 

 have been liberated in cracks within the rock, they have flowed on, 

 excavating channels for themselves. It is only when the cavern 

 admits of a certain amount of evaporation taking place that the 

 erosive action begins to be counteracted by the deposit of the dis- 

 solved material in pendent masses descending from the roof of the 

 chamber. These stalactites may grow, like columns, until the cavern 

 is filled with them ; for the drippings from the stalactites often form 

 corresponding masses on the floor termed stalagmites, which rise up to 

 meet the descending pillars. Many, however, of the most interesting 

 caverns have been filled up with mechanical deposits before the 

 stalagmitic covering was developed ; and in such gravels or red cave- 

 earth the remains of fossil mammals occur. The chief British caves 



