550 REPORT OF NATIONAL MUSEUM, 1893. 



greenish layers, the brown layers contaiuiiig it in the form of both 

 protoxide and more or less hydrated sesqnioxides. , Bischotf, in speak- 

 ing of a like condition in the so-called sprudelstein of Carlsbad, says:* 



The brown coutaiiis a considerably larger (juautity of peroxide of irou than the 

 white, Avhich is sometimes quite free from it. This difference jtresupposes either 

 that there is a difference in the quantity of irou contained in the water, or that 

 sometimes the atmospheric air has a greater influence than at others, and that in 

 those cases a larger quantity of protoxide of iron is peroxidized. 



In the case of the stones here described the percentage of iron in the 

 green and brown and red varieties is nearly the same, the varying hues 

 depending mainly upon its chemical condition. While it is possible 

 that a part of the change from protoxide carbonate to sesquioxide 

 took place at the time of deposition, a large part is due to oxidation 

 which has taken place since the beds were in substantially their present 

 condition, and is due to i^ercolating solutions. That this is the case is 

 abundantly proven by the fact that the oxidation in most cases can be 

 readily seen to have iirogressed along lines of Jointing and fracture 

 and along the more porous layers. In many cases the oxidation is 

 accompanied by a partial removal of the lime carbonate, whereby the 

 stone is rendered cellular and unfit for use. Such, however, is not 

 always the case, and many of the oxidized varieties are beautiful in the 

 extreme, as well as uni(jue. {^See under chemical and physical proper- 

 ties, p. 558.) 



The cave marbles differ from the travertines mainly in method of dep- 

 osition, being cold-water deposits irpon the walls and floors of limestone 

 caves. Eain water passing through the atmosphere and soaking 

 through the layer of soil by which the earth is covered becomes charged 

 with a varying amount of carbonic acid, which gives it the power of 

 dissolving slowly the lime carbonate forming the essential constituent 

 of the rock limestone, as already noted. Filtering downward through 

 cracks and fissures or between the laminje com])osing the beds, it thus 

 gradually enlarges them until what are popularly known as caves or 

 caverns are produced. But after this cave-forming process has gone 

 on for awhile another process sets in, whereby the cavern may be wholly 

 or in part refilled. The water from the surface percolating down through 

 the roof of the cave dissolves out a portion of the lime carbonate, just 

 as when running through a crack or fissure, but in this case the 

 water comes through the overlying rock and renmins for a time sus- 

 pended, in the form of a drop, from the ceiling. Here it evaporates or 

 loses a part of its carbonic acid, and, unable longer to hold the lime in 

 solution, begins to deposit it in the form of a ring around the outer mar- 

 gin of the drop. As time goes on this ring becomes prolonged into a 

 quill-like tube, growing in length always from its lower end. After a 

 time, as a rule, this frail tube becomes partially or wholly closed, when 

 the water flows down over the outside, the growth now being wholly 



^ Op. cit. p. 148. 



