CONDITIONS FAVORABLE TO INJECTION; 649 



heating of the solutions by the igneous rocks. In the belt of weathering 

 the range of action of the gaseous solutions is somewhat limited. (See 

 pp. 430-431.) In the belt of cementation the range of action of the liquid 

 solutions may be far more extensive and the total result vastly greater. 

 Indeed, I believe it would be difficult to overestimate this indirect effect. 



The solutions are the great agents which permeate all the openings of 

 the rocks, great and small, travel great distances from the source whence they 

 derived their heat and dissolved materials, and in their journey everywhere 

 alter the rocks. 



To illustrate, the waters of hot springs, such as those of the Yellowstone 

 Park, derive their heat from igneous rocks at considerable distances from 

 the points where the waters issue, and throughout the journey the activity 

 of these solutions is very greatly enhanced, and therefore exomorphism is 

 greatly accelerated. 



The amount of contact action and the distance to which it extends 

 depend mainly upon (a) the porosity of the intruded rock, (b) the efficiency 

 of the water circulation, (c) the composition of the intrusive and intruded 

 rock, (d) the size of the intrusive masses, (e) the length of time of intrusion, 

 (f) the temperature of the intrusive, and (g) the depth of the intrusion. 



(a) A most important factor in the extent and amount of contact 

 metamorphism is the porosity of the intruded rock. If the rock be 

 nonporous the contact effect is usually limited to a somewhat narrow belt 

 adjacent to the intrusive. If the intruded rock be broken by faults, joints 

 planes of fissility, and especially if the individual masses between the larger 

 openings are porous, as in the case of sandstones, the contact effect for a 

 given intrusive mass may extend many times farther and be many times 

 more effective than in a nonporous rock. 



Chemical change is greatly facilitated by the perviousness of the 

 intruded rock. A porous rock, such as a sandstone or a fissile rock, has 

 openings in which may be deposited great additions of materials different 

 from the unmodified rock, and thus change its chemical composition. For 

 instance, a quartzose sandstone may have large additions of feldspar or 

 amphibole, as in the Wausau district of Wisconsin. On the other hand, an 

 impervious rock, such as a mudstone or shale, may be but little changed, as 

 the solutions can not readily penetrate this rock and carry with them 



