650 A TREATISE ON METAMORPHISM.' 



constituents of a different composition. Of course an impervious rock may 

 be considerably changed in chemical composition immediately adjacent to 

 the intrusive, but the change is not likely to extend far. (See e.) 



(b) If circulating underground waters are in rapid motion in the 

 intruded rock — and this of course depends, among other things, upon the 

 porosity — the contact effect will extend widely and the alterations be great, 

 for the rapidly moving percolating waters derive both heat and soluble 

 material from the igneous rocks, or the heated rocks adjacent to them, and 

 this material and the heat are carried far and therefore produce great 

 effects. 



(c) The amount of contact action depends upon the compositions of 

 the intrusive and intruded rocks. The general law in this connection is that 

 the exomorphism and endomorphism tend to make the rocks approach each 

 other in chemical composition. Therefore the amount of change is likely 

 to be great in proportion as the difference in composition of the intrusive 

 and intruded rocks is great. If they be practically the same in chemical 

 composition, the interchange of constituents through the solutions is not 

 likely to greatly modify the composition of either. But if, on the other 

 hand, the two rocks differ greatly in composition, each will be likely to gain 

 constituents from the other and be modified, in chemical and mineral compo- 

 sition. Each furnishes to the other materials in which it is deficient. 

 Thus, if an intrusive rock in which feldspar and bisilicates are abundant, 

 but in which quartz is absent, intrudes a rock rich in quartz, but one in 

 which feldspar and bisilicates are absent, the intruded quartzose rock will 

 be likely to gain feldspar and bisilicates, while the intrusive rock will be 

 likely to gain silica, and may gain an amount sufficient to cause the silica 

 to separate as quartz. An excellent example of the addition to the intruded 

 rock from the intrusive is furnished by the adinoles and spilosites of the 

 Crystal Falls district of Michigan described by Clements. 11 These metamor- 

 phosed slates contain very much more potassium than the average slate of 

 the district. The igneous rock is rich in potassium and this element, there- 

 fore, must have been derived from that source. 



In producing the above effects intrusive rocks are likely to cause 

 important change in proportion as they are mobile and in proportion as 

 they contain a large amount of occluded water. Indeed, to a certain extent 



« Clements, J. Morgan, A contribution to the study of contact nietamorphism: Am. Jour. Sci., 4th 

 ser., vol 7, 1899, pp. 85-90. 



