PROPERTIES AND USES OF CELL WALLS 107 



(c) The relative importance of thermal processes in formation of 

 lower rank coals is disputed and questionable, but the formation 

 of anthracite reportedly requires temperatures of 350°C-600°C. 

 Locally, igneous intrusions may provide heat for such transforma- 

 tions, but the temperature in regional deposits was probably 

 elevated by frictional heat generated at shear surfaces. The generation 

 of pressure is obviously associated with these regional heating 

 effects. 



Little is known with certainty but a great deal has been inferred 

 about the details of coal formation. The general features of the 

 coalification process and its relation to cell wall substances are 

 reasonably well established, as should be evident even in this 

 brief discussion. 



