106 AQUEOUS EOCKS 



Stalactite and stalagmite are the names given to the deposits 

 formed from the roofs and on the floors of caves; water, perco- 

 lating through the hmestone roof, by virtue of the carbonic acid 

 it contains, dissolves out a small amount of the lime, which, on 

 evaporation, is again deposited either as pendent cones from 

 the ceiling, or as massive and pillar-like forms upon the floor. 

 The pendants are known as stalactites; the corresponding 

 growths upon the floor as stalagmites. Stalactite and stalag- 

 mite sometimes meet, forming thus continuous pillars, or col- 

 umns extending from floor to ceiling. The lime of these 

 deposits, it may be said, is as a rule in the form of calcite, 

 though sometimes, as in the old portions of the Wyandotte 

 caves in Indiana, it is aragonite. The so-called '^ oriental ala- 

 baster'' of the ancients is a stalagmitic deposit derived in part 

 from crevices and pockets in the Eocene limestones of the Nile 

 valley. 



Magnesite, a carbonate of magnesia, occurs frequently as a 

 secondary mineral in the form of veins in serpentinous rocks, 

 but rarely itself forms rock masses of any importance. Ehodo- 

 chrosite, a carbonate of manganese, sometimes occurs in rock 

 masses, but is found most commonly in the form of veins asso- 

 ciated with ores of silver, lead, or copper. 



Another carbonate, less common than that of lime, but which 

 sometimes occurs in such quantities as to constitute true rock 

 masses, is stderite^ or carbonate of iron. A common form of 

 this is dull brownish or nearly black in color, very compact and 

 impure, containing varying amounts of calcareous, clayey, and 

 organic matter. In this condition it is found in stratified beds 

 and in the shape of rounded and oval nodules, or concretions, 

 which are called clay4ro%stone nodules, septaria, and spkcero- 

 siderite, (See Fig. 2, PL 8.) These septarian nodules are 

 often beautifully veined with calcite, and when cut and polished 

 form not undesirable objects of ornamentation. 



(3) SILICATES 



Silica, combined with magnesia and water, gives rise to an 

 interesting group of serpentinous and talcose substances, which 

 are often sufficiently abundant to constitute rock masses. Pure 

 serpentine consists of about equal parts of silica* and magnesia, 

 with from 12 to 13% of water. It is a compact, amorphous, or 

 colloidal rock, soft enough to be cut with a knife, with a slight 



