314 ELEMENTARY GENERAL SCIENCE CHAP. 



one variety shows a well-developed iridescence, and is called 

 precious opal. 



Rocks formed of Silica. Besides these minerals some rock- 

 masses are composed of silica. Of these, as in the case of 

 calcium carbonate, some have been deposited chemically, others 

 by plant life or animal organisms. 



Sinter is the rock resulting from the deposition of silica dis- 

 solved in the water of hot springs and geysers. It is, however, 

 probable that, even in this case, minute plants of the Alc/ce 

 group assist in the formation. 



The organically formed rocks of silica may be divided into : 



(1) Those formed &?/ Plants. These plants also belong to the 

 Algse group and are known as the diatoms. They are generally 

 microscopic in size, but have existed in sufficient numbers to form, 

 by the accumulation of their hard remains, beds of considerable 

 thickness. They arc made of the silica which the living diatom 

 secreted from the water, either fresh or salt, in which it lived. 

 At Richmond, in Virginia, beds, of forty feet thick, and consist- 

 ing entirely of diatoms, occur. Diatom rirpous earths and Tripoli 

 powder have been made in this way. 



(2) Those formed by Animals. These silica- secreting animals 

 belong to the class of simplest animal structure known ; they 

 are called Eadiolana. Their remains build up the Ef(diularin. 

 earths which occur in various places, notably Barbados. 



Chemically Prepared Silica. When chemically prepared, 

 silica, or silicon dioxide, is obtained as a white powder, which is 

 insoluble both in water and in most acids. If fused with caustic 

 soda, however, it yields a glassy mass, which is soluble in water 

 and is known as water glass. Chemically it consists of sodium 

 silicate, Na 2 SiO 3 ; in this, as in the natural silicate, the silica plays 

 the part of an acid oxide. 



As a rule the silicates are insoluble and their composition is 

 very complicated. Little is known concerning their exact chemical 

 nature. Amongst the commoner silicates may be mentioned clay, 

 a silicate of aluminium ; felspar, a silicate of aluminium and 

 potassium; serpentine, talc, French chalk, and meerschaum, 

 silicates of magnesium ; and asbestos, a silicate of calcium and 

 magnesium. 



