INTERACTION OF INTRUSIVE AND INTRUDED ROCKS. 713 



masses are introduced into sedimentary, volcanic, or other rocks, which 

 have passed through the zone of katamorphism, they already contain a 

 reservoir of water in hydrated minerals which can be released by dehy- 

 dration. This supply is usually ample to entirely recrystallize the rocks. 

 But as nearly as we can ascertain from observation, the magmas themselves 

 contain a considerable store of water, as pointed out on page 662. At the 

 time of crystallization of a magma a portion of this water escapes and passes 

 slowly into the surrounding rocks, and thus in magmas we have a second 

 reservoir, which supplements the first; so that adjacent to masses of igneous 

 material in the zone of anamorphism there is commonly an amount of water 

 such that recrystallization readily takes place. 



COMPOSITION OF INTRUSIVE AND INTRUDED ROCKS. 



The absolute and relative compositions of the intrusive and the intruded 

 rocks are both of importance in reference to the alterations which take 

 place. So far as observation can determine, it seems to be a fact that the 

 alterations are more widespread in connection with batholiths of granite 

 and syenite and other acid rocks than with basic rocks. Possibly the 

 apparently greater contact effect may not be due so much to the chemical 

 composition of the crystallized intrusive as to a greater than average 

 amount of occluded water (see pp. 720-728), and to the high temperatures 

 which are frequent accompaniments of acid magmas. 



The character of the intruded rock has much to do with the nature of 

 the alteration. In so far as rocks are composed of minerals which are easily 

 recrystallized, this process is more likely to take place; whereas rocks 

 which are difficult to recrystallize under the same conditions may be com- 

 paratively little altered. The presence of hydrous minerals which may 

 be dehydrated and furnish water, and fineness of grain are very favorable 

 to recrystallization. 



As to the mutual effect upon the chemical composition of the intrusive 

 and the intruded rocks, the law in the zone of anamorphism is the same as 

 that in the belt of cementation, viz, that the joint endomorphic and exo- 

 morphic effect is to make the intruded and intrusive rocks approach each 

 other in composition. Under the law of mass action each gains from the 

 other elements in which it is deficient. One of the commonest illustrations 

 of this law is furnished by the large amounts of feldspar which frequently 



