The Scottish Naturalist. 



153 



ingredient in mortar, or its use as an element in the manufacture 

 of glass. 



In looking closely at a handful of sand, we observe that it is 

 composed chiefly of small grains of a transparent, glassy-like 

 substance. This is Quartz— -a term used by the miners in Ger- 

 many to denote rock-crystal, or crystalline silica. 



Silica, or Silex, is a simple or elementary body, little known in 

 a pure state, but, when united with oxygen, well known as Silica or 

 Silicic acid. 



Rock crystal, topaz and cairngorm stone (smoky quartz), jasper 

 and amethyst, chalcedony and agates, opal and carnelian, are all 

 familiar members of the quartz family. Their colours and forms 

 are due to union or combination with other substances, e.g., jasper, 

 composed of quartz, alumina and the peroxide of iron, has its 

 colour from the presence of the latter, while the amethyst owes 

 its beautiful colour to the oxide of manganese. 



Besides occurring in these forms, quartz is common as an 

 amorphous or shapeless rock ; and as a penetrating vein or lode 

 in other rock-masses. It is the most frequent matrix of gold 

 or tin deposits. 



Silica enters into combination with most mineral substances. 

 United to oxygen, it forms a large part of the crust of the globe. 

 It is to the free presence of silica that the quill part of birds' 

 feathers, the shields of certain infusoria and the spicula of the 

 greater number of sponges, are indebted for their rigidity. The 

 straw of cereals owes its firmness and stiffness to silica absorbed 

 by the plants from the soil, and deposited on the outside of their 

 stems or stalks. The joints of the bamboo have it largely de- 

 posited in their interior. These plants are thus furnished with an 

 element in their structure which serves the same purpose that 

 bones do in men and animals, and without which they could not 

 maintain an erect position. 



Another substance frequently present in sand is felspar or rock- 

 spar — the name is derived from the German word feldspath. 

 Felspar is an important and abundant constituent of rocks ; and 

 consists mainly of silica and alumina (the basis of clays, loams, 

 and other argillaceous earths), with potash or soda, and traces of 

 lime, magnesia and the peroxide of iron. Like quartz it is found 

 crystallized, amorphous, and disseminated. It is of different 

 degrees of hardness ; and is used for a variety of purposes. The 



