68 OUT I.I M- OF Ml LD-GEOLOGY 



powder made by a scratch with the- knife. Of course, 

 individual minerals which occur cither as original or 

 accidental constituents of rocks may be tried for I 

 in the usual way required in mineralogical inquiry. 

 Small specks of haematite may thus be detected by their 

 characteristic cherry-red streak, while the iron-peroxide 

 when hydrated (limonite) will show its brown or yellow 

 ik. 



4. Colour. Great caution must be exercised in 

 making use of this character in the discrimination of 

 rocks. The same rock may, even within short distances, 

 display the most extraordinary varieties of colour. But 

 within certain limits the colour of a rock is an indication 

 of the nature of some at least of its constituents. Iron is 

 the great pigment to which the rocks owe their diversities 

 of hue. It gives rise to numerous tints of yellow, brown, 

 red, and green, as well as to blue and black. Some 

 hints as to the causes of a few common varieties of colour 

 may be of service. 



II 'hite. Limestones and clays are often quite white, 

 and in this condition are almost always at their purest. 

 Iron is generally absent, or present in but small quantity, 

 in white rocks. The result of weathering is often to 

 bleach rocks white, the air and rain removing the colour- 

 ing materials, more especially the iron. The stones in a 

 morass, or below peat are commonly bleached on the 

 outside as white as chalk the result of the reducing 

 action of organic acids from the peat on the iron oxides, 

 which are removed in solution or suspension, as organic 

 compounds or as carbonates. 



Black. Many carbonaceous rocks are black. Coals 



