INTERACTION OF INTRUSIVE AND INTRUDED ROCKS. 715 



As just noted, where the contacts are sharp one does not need to go 

 far in order to obtain rocks the chemical composition of which corresponds 

 to that of the intruded and intrusive masses, although the intrusive may be 

 affected to a greater distance by the absorbed material than are the intruded 

 rocks, since in a magma the elements are more rapidly distributed than 

 they are in the solid rock, in which the water is very small in amount and 

 moves very slowly. However, even in the case of the intruding' magma, 

 Becker a and others have shown that the process of diffusion is compara- 

 tively slow, a very long time being required for the chemical composition of 

 the intrusive to become greatly affected at a considerable distance from the 

 contact with the intruded rock. 



Where, however, the intruded and intrusive rocks are intimately min- 

 gled there may be profound modification of the chemical composition of 

 both rocks. A case of this already given is that of parallel injection into 

 slates, schists, and gneisses. In such a case the modification may be so 

 great that the chemical composition of the intruded rock, if sedimentary, 

 may vary greatly from that of ordinary sediments, and the intruded rock 

 may differ considerably in composition from the main mass of inirusives. 

 Such rocks as these are the so-called injection gneisses (See pp. 725—727.) 

 These rocks are usually coarsely or finely banded, and may be aqueous 

 and igneous in any proportion, or igneous rocks cf two kinds in any pro- 

 portion, provided the intruded rock was a schistose igneous one. 



Another case of intimate mingling of the two rocks is that where 

 numerous fragments of the intruded rock are caught by the intrusive. By 

 subsequent movement such fragments may make their way into the magma 

 for some distance from its border, and their partial or complete absorption 

 may affect the chemical and mineral composition of the intrusive for con- 

 siderable or even long distances from the outside of the intrusive. 



In closing this part of the subject it should be noted that the above 

 statements as to the rather short distances to which chemical changes are 

 usually limited does not contradict the statement already made, and more 

 fully developed later, that the metamorphic effect of great intrusives often 

 extends far from the intrusive masses. 



"Becker, George F., Some queries on rook differentiation: Am. Jour. Sci., 4th ser., vol. 3, 1897, 

 pp. 27-31. 



