530 REPOKT OF NATIONAL MUSEUM, 1890. 



beautifully veined and colored by metallic oxides and form some of the 

 finest marbles (Specimen 39071 from Mexico; 38445 from Suisin City, 

 California; 37269, Idaho ; and 388L1, Tivoli, Rome, Italy. See. also, col- 

 lection of building and ornamental stones). 



Stalactite and stalagmite are the names given to the deposits formed 

 from the roofs and on the floors of caves ; shown by specimens from 

 the rock of Gibralter (36769 and 38444) and from the Luray Caves in 

 Virginia (35549). See also floor upright case on north side of hall. 



Magnesite, a carbonate of magnesia, occurs frequently as a secondary 

 mineral in the form of veins in serpentinous rocks (specimens 70158 

 from Lancaster County, Penn sylvania; 70678, Wells Island, New York; 

 and 28464, Victoria, Australia). 



Rhodochrosite, a carbonate of manganese, sometimes occurs in rock 

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

 with ores of silver, lead, or copper (specimen 26745 from Walkerville, 

 Montana). Another carbonate, less common than that of lime, but 

 which sometimes occurs in such quantities as to constitute true rock 

 masses, is siderite, 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 con 

 dition it is found in stratified beds and in the shape of rounded and 

 oval nodules, or concretions, which are called "clay-iron-stone" nodules, 

 "septaria," and " sphserosiderite" (specimen 12840 from Wakonda, Kan- 

 sas ; see, also, Fig. 2, PI. cxxn). These septarian nodules are often 

 beautifully veined with calcite (see concretion collection). Other forms 

 of siderite, like those from Connecticut (36105) and Saxony (3810 aud 

 39073), are massive, coarsely crystalline, and of a nearly white or yel- 

 lowish color, becoming brownish on exposure. Pure siderite yields 

 about 48 per cent, metallic iron, and is a valuable ore. As with the 

 other ores of iron, but a few characteristic specimens are here exhibited, 

 and the visitor is referred, as before, to the ore series for a more complete 

 display. 



(3) Silicates. — Silica (oxide of silicon) 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 mag- 

 nesia, with from 12 to 13 per cent, of water. It is a compact, amorphous 

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

 feeling and luster, and of a color varying from dull greenish and al- 

 most black, through all shades of yellow, brownish, aud red. It also 

 occurs in fibrous and silky forms, filling narrow veins in the massive 

 rocks, and is known as amianthus, or chrysolite. These fibers, when 

 sufficiently long, are used for the manufacture of fireproof material, and 

 the mineral is commercially confounded with asbestus, a fibrous variety 

 of hornblende (specimen 37645 from Canada). It is very doubtful if 

 serpentine is ever an original rock, but is always derived from the altera- 

 tion of other and less stable magnesiau minerals, Here are exhibited 



