THE ONYX MARBLES. 551 



external. In this way are fornieil the elongated pendant cones from 

 the roofs of caves, and to which the name stalactite is given.* Such 

 on being cut and i^olished show a beautiful zonal structure, not wholly 

 unlike the rings of growth upon the trunk of a tree. (PI. 13.) 



But it rarely happens that all the water evaporates upon the ceiling; 

 a portion usually falls upon the floor, where by a similar process it 

 builds up a deposit chemically the same as the stalactites, but difteriug 

 in that owing to the spreading out of the water as it falls, the floor 

 deposits are more massive in form. To these floor deposits the name 

 stalagmite is given. In some cases they rise in the form of blunt 

 trunks or cones to meet their corresponding stalactites above until 

 there are formed continuous pillars from floor to ceiling, as shown in 

 l)late 1. If this process goes on for a suflicien t time the entire cave may 

 be refilled, and since the water in percolating through the roof dissolved 

 only the pure lime carbonate, or with but a trace of impurity, leaving 

 nearly all the carbonaceous, siliceous, and clayey constituents behind, 

 so these stalactitic and stalagmitic deposits are of purer lime, refined by 

 nature's methods and recrystallized under new conditions. The form 

 of crystallization, it should be stated, is sometimes that of aragonite, 

 but so far as the writer's experience goes, more commonly that of 

 calcite. It is sometimes, though not always, possible to distinguish 

 between the two forms of crystallization by the unaided eye, stalac- 

 tites (or stalagmites) of aragonite showing interiorly a radiating 

 fibrous structure, the fibers being not infrequently beautifully curved 

 and of a silky luster, while those of calcite are more granular. It some- 

 times happens that deposits of both kinds are to be found in the 

 same cave, though so far as my own observation goes they belong in 

 such cases, as at Wyandotte, Indiana, to diftereut periods of growth. 

 What the conditions are upon which these varying forms of crystalli- 

 zation dei^end is not now apparent. 



It follows almost from necessity from their mode of origin, as above 

 given, that the beds of onyx marbles, both spring and cave deposits, are 

 as a rule lar less extensive and regular in their arrangement than are 

 the ordinary stratified and bedded limestones and marbleSc Spring 

 action is more or less intermittent, and the |)Iace of discharge, as well 

 as the character of the deposit, variable. The latter usually take the 

 form of a comparatively thin crust, conforming to the contours of the 

 surfaces on which it lies. The various layers thicken aud thin out 

 irregularly, and are often lenticular in cross section. Sound and homo- 

 geneous layers of moi-e than 20 inches thickness are not common. 

 Where two or more layers, of sound and merchantable material occur 

 they are not infrequently separated by tufaceous material, foreign 

 debris, or by impure aud porous onyx of little value. This condition 

 of aft'airs will become more apparent as particular occurrences are 



* On tlie Fonuatiou of Stalactites aud Gypsnm lucrustatious iu Caves. Proc. 

 U. S. Nat. Muiseuui, xvii, 1894, p. 77. 



