220 INDUSTRIAL MINERALS DIAGRAM 1 9 



mineral kingdom are not less numerous or important than those 

 derived from 'the animal and vegetable kingdoms. 



Granite. The primitive formations are chiefly composed of 

 granite. It is found at the surface of the ground in many 

 places, only cohered by a thin layer of vegetable soil. 



Granite is generally considered the hardest and most durable 

 of stones. There are certainly some kinds of granite which pos- 

 sess these qualities, but all do not possess them in the same 

 degree, and there are some kinds of granite which deteriorate 

 very easily. Hard granite is used for engineering purposes, but 

 does not answer so well for architecture. It is sometimes rose- 

 coloured, and sometimes bluish or black. The working of this 

 stone is always somewhat difficult, firstly on account of its hard- 

 ness, and secondly because it exists like all primitive rocks, 

 in enormous masses which are more difficult to deal with 

 than rocks, which are perhaps quite as hard, but which are ar- 

 ranged in layers one above another. 



Fossils are never found in granite. 



Pumice-stone. Pumice-stone is a volcanic product, and is only 

 found where volcanoes exist or have existed. It is a stone re- 

 markable for its porous structure, which causes it to float in 

 water on account of the air contained in its cavities. Pumice- 

 stone is very brittle and friable, but at the same time very hard. 

 It is reduced to powder, and used in industry to polish wood and 

 ivory, as well as leather and parchment. 



Sulphur is another volcanic product which is found under the 

 same circumstances as pumice-stone, but sulphur is of much 

 more extensive use in industry. It is employed to make gun- 

 powder, mixed with 'charcoal and saltpetre: It is used for 

 matches, and is also employed in the manufacture of sulphuric 

 acid or oil of vitriol, great quantities of which are used in. indus- 

 try. Lastly, powdered sulphur is also used by agriculturists to 

 destroy microscopic fungi on plants. The sulphur gathered near 

 volcanoes is sometimes very pure, and is then called native sul- 

 phur. But it is generally mixed with earth, and must be puri- 

 fied before it is fit for use. Sulphur burns in air with a blue 



