102 THE PROSPECTOR'S HANDBOOK. 



Varieties : 



Slate Clay Colour, greyish or greyish yellow. Frac- 

 ture slaty. When ground and reduced to paste with 

 water can be used as firebrick. 



Common Clay (used for making bricks, tiles, and coarse 

 pottery). Loam. 



Pipe Clay Colour, white or greyish white ; feels greasy. 

 Surface polishes when pressed by the finger. 



Potters' Clay More easily fusible. Of various colours ; 

 generally red, yellow, green, blue, &c., becoming red 

 or yellow when burnt. 



Kaolin (porcelain clay). The purest form of clay. 

 Contains 40 to 42 per cent, alumina, 46 to 48 per 

 cent, silica, and water. It is really decomposed fel- 

 spathic rock. Kaolin is greasy to the touch, friable in 

 the hand, and does not easily form into a paste with 

 water. When heated, hardens and retains a white 

 colour. 



NATURE OP CERTAIN MINERALS 



Met with in various of the Igneous and Metamorphic Rocks. 



Quartz (see Matrices). 



Felspar. 



Colour usually white or red, occasionally grey, black, or 

 green. Felspars scratch glass and can be scratched by 

 quartz ; but not well by a good knife. S.G. 2-5 to 2'7. 

 Lustre commonly vitreous or pearly on the more perfect 

 cleavage planes. Some varieties are iridescent or opal- 

 escent. With the exception of Labrodorite, felspars are 

 unacted on, or imperfectly so, by acids. Contains silicate 

 of alumina with soda, potash, lime (sometimes two or more 

 of these). 



Mica. 



A finely foliated mineral of pearly lustre. In'colour some- 

 times white, grey, or black, and when exposed sometimes 

 yellowish. Cleavage very perfect in one direction. Laminae 



