EFFECTS OF HEAT AND LIGHT. 55 



The speed of nietainorphisin is therefore largely dependent upon the amount 

 of heat and light present, especially the former. 



In rock alteration heat and light produce direct effects and indirect 

 effects. 



DIRECT EFFECTS OF HEAT AND LIGHT. 



The more important direct effects may be either mechanical or 

 chemical. 



Mechanical effects. — The mechanical effects are desiccation, baking, and 

 fusion. At the surface of the earth the heat of the sun frequently results 

 in evaporating the moisture and desiccating the rocks; an attendant result 

 is induration. This process is especially important in the clay sediments, 

 and occurs to the greatest extent in the hot and arid regions, although 

 desiccation is not unimportant in the colder regions. The details of the 

 process especially concern the belt of weathering and are treated in the 

 chapter on that subject. (See Chapter VI, pp. 541-550.) Where igneous 

 rocks as a consequence of volcanism are brought into contact with other 

 rocks the latter may be baked for a longer or shorter distance from the 

 igneous rocks. The process of baking as here used is restricted to modifi- 

 cations similar to those which take place in the baking of bricks; that is, to 

 effects which are mainly due directly to the heat. This process is restricted 

 to the belt above the level of underground water — the belt of weathering — 

 and is therefore treated in detail in the chapter on that subject. (See 

 Chapter VI, pp. 488-494.) Below the belt of weathering' the rocks are 

 saturated with water and the heat effects are mainly produced through that 

 agent. Even in the belt of weathering the baking effect is not wholly due 

 to heat, but is partly accomplished through the agency of the contained 

 water, precisely as is the transformation of clay to brick by burning; for all 

 rocks under natural conditions contain gas and water, and usually consid- 

 erable quantities. During the baking process the original molecules are 

 brought nearer together, but there are also important chemical changes. 



Where the masses of the igneous rocks are very great, and especially 

 where adjacent rocks are included in masses of igneous rocks, the rocks 

 may be softened by the heat or even absorbed by the magma. Where 

 the rocks are softened they are likely to be very greatly changed, perhaps 

 recrystallized. Where they are absorbed by the magma they are lost as 

 original rocks and become a part of the magma by which they are absorbed. 



