ON THE FORMATION OF CAVITIES IN ROCKS. 21 



Mammoth Cave and Luray's Cave) are excavated. Caves are, 

 however, almost always due to surface waters, and do not extend 

 below the permanent water level unless they have been depressed 

 after their formation. (See J. S. Curtis, Monograph VII., U. S. 

 Geol. Survey, Chap. VIII.) 



The solvent action of water is vastly augmented by the car- 

 bonic acid which it gathers from the atmosphere, and this is the 

 chief cause of the excavations wrought by it in limestones. Pure 

 cold water has comparatively small dissolving and almost no ero- 

 sive power. It has also been advocated that various acids which 

 result from the decay of vegetable matter aid in such results. (A. 

 A. Julien, Amer. Asso. Adv. Sci., 1879, p. 311.) This may be true, 

 but in general carbonic acid is the chief agent. Iron in minerals 

 falls an easy prey, as well as calcium, and is dissolved out in large 

 amount. (See Example 1.) When charged with alkaline carbon- 

 ates, water has the power to attack other less soluble minerals, such 

 as quartz and the silicates, and by such action the walls of a 

 cavity in the crystalline rocks may be much affected. 



1.02.13. Waters percolating to great depths in the earth, or 

 circulating in regions of igneous disturbances, become highly 

 heated, and this too at great pressure. Under such circumstances 

 the solvent action is very strongly increased, and all the elements 

 present in the rock-making minerals are taken into solution. 

 Alkaline carbonates are formed in quantity ; silica is easily dis- 

 solved ; alkaline sulphides result in less amount ; and even the 

 heaviest and least tractable metals enter into solution, either in the 

 heated waters themselves, or in the alkaline liquors formed by them. 

 The action on the walls of cavities and courses of drainage is thus 

 profound, and accounts for the frequent decomposed character of 

 the walls and the general lack of sharpness in their definition. 

 The vast amount of siliceous material, etc., deposited by hot springs 

 and geysers is additional evidence of its importance. 



Magnesia is one of the alkaline earths readily taken into solu- 

 tion by carbonated waters, and when such waters again meet lime- 

 stone the effect is often very great, and constitutes one of the 

 most important methods of the formation of cavities. Solutions of 

 magnesium carbonate, on meeting calcium carbonate, effect a par- 

 tial exchange of the former for the latter. This leaves the rock a 

 double carbonate of calcium and magnesium, which is the composi- 

 tion of the mineral and rock dolomite. The process is therefore 

 called dolomitization. (See Example 25.) It may bring about a 



