CONDITIONS FAVORABLE TO INJECTION. 651 



these two factors are mutual ; for the greater the amount of occluded water 

 the greater the mobility of the magma. The greater the mobility the more 

 extensive the intrusion is likely to be and the more minute the openings 

 which may be entered. Further, magmas which contain a large amount 

 of occluded water at the time of crystallization emit this water loaded with 

 salts, and this material is added to the intruded rocks and increases the 

 exomorphism. 



(d) Other things being' equal, the greater the mass of the intrusive the 

 farther its effects are likely to extend and the more potent they are likely 

 to be. While there is a tendency for the metamorphism to be proportional 

 to the size of the intrusives, the small metamorphic effect upon the adjacent 

 rocks which large intrusive masses, such as the great dikes or even the 

 laccoliths, frequently show is astonishing. In many cases the intruded 

 rocks seem to be scarcely modified at all by masses of the largest size in 

 the belt of cementation. An excellent illustration of great intrusive masses 

 with little or no apparent metamorphism is furnished by the extensive sills, 

 from 10 to 100 or more meters thick, in the Animikie slates northwest of 

 Lake Superior. But, as explained below, comparatively small masses of 

 intrusive rocks may, for various reasons, result in profound modification. 



(e) An important factor in the extent of metamorphism is the length 

 of time through which flow continues along a certain opening. If mag- 

 matic flow continues long through an opening, the quantity of passing 

 magma is great and the contact effect is likely to extend far and to be 

 profound. This is a general consequence of the large amount of heat and 

 emanations which may be furnished to the surrounding rocks and solutions 

 by the long'-continued intrusions. Long-continued flow of lava to higher 

 levels or to the surface may take place through small openings, and a large 

 intrusive mass may quickly form. Therefore a comparatively narrow 

 igneous mass may be accompanied by profound contact action, because 

 the changing mass furnishes to the intruded rock a continuous supply of 

 heat and material to the solutions for a long time; while accompanying- 

 great masses of igneous rocks which simply occupied an area, compara- 

 tively little contact action may occur. The latter case is illustrated by the 

 Henry Mountain laccoliths ° and the great dolerite sills, some of them 



Gilbert, G. K., Report on the geology of the Henry Mountains: U. 8. Geog. and Geol. Surv., 

 Rocky Mt. Region, 1880, pp. 51-9S. 



