CAUSES OF METAMORPHISM 29 1 



pal agents of the changes, though igneous intrusions frequently 

 add very materially through the extensive development of contact 

 metamorphism. 



The igneous rocks, when subjected to the same processes, give 

 rise to rocks similar to those made from the metamorphism of 

 sediments. The compression, shearing, and crushing may take 

 place while the molten mass is still pasty, or long after the rock 

 has cooled. Certain rocks have been formed from the meta- 

 morphism of sediments and the injection of igneous material, and 

 are thus of highly complex origin. 



III. The Causes of Metamorphism 



This is a subject which bristles with difficulties, and of which 

 our knowledge is yet very incomplete, though, in a general way, 

 the agencies of the transformation are intelligible. 



(1) Heat is evidently a very important factor of the change. 

 This is made plain by the phenomena of contact metamorphism 

 and by the numerous successful attempts to imitate metamorphism 

 experimentally. On the other hand, it is not believed that high 

 temperatures are always indispensable. Change in which heat is 

 the principal factor is called thermal metamorphism. 



(2) Compression is the principal agency in regional meta- 

 morphism, and to it are due the structures of cleavage, fissility, 

 and schistosity, as well as the reconstruction and crystallization of 

 mineral particles. This is dynamic metamorphism, but heat is 

 probably a common accessory in this method of change also. 

 To be in the zone of flowage, rocks must be so deeply buried 

 that they are invaded by the earth's internal heat ; and unless the 

 movement be excessively slow, flowage must generate frictional 

 heat. 



(3) Moisture is likewise a potent cause of change. Under 

 pressure, water may be heated to very high temperatures, when it 

 becomes capable of attacking and dissolving or decomposing the 

 most refractory substances, and building them up into new com- 

 pounds. Many minerals, such as orthoclase and quartz, which 



