1016 A TREATISE ON METAMORPHISM. 



develop. This heating- of solutions in the surrounding strata by igneous 

 rocks is believed to be a factor of the most pronounced importance in the 

 segregation of many ore deposits. 



Fourth. The introduction of the igneous rocks may promote fracturing 

 and thus open channels for circulation. Also, after an igneous rock has 

 crystallized it continues to cool and to contract. In consequence of its con- 

 traction trunk channels maybe opened along the contact of the intrusive and 

 intruded rock and within the igneous mass. The production of openings is 

 very favorable to the segregation of ores. 



Fifth. Another important function of the igneous rocks is to furnish 

 impervious basements which control the trunk channels of circulation, as, 

 for instance, in the Lake Superior region, in the Leadville and Tenmile 

 districts of Colorado, and in the Mercur district of Utah. 



The formation of a given ore deposit with the aid of igneous rocks 

 may be a consequence of any combination of the above factors, and the 

 relative importance of each factor is variable in different cases. In a given 

 district the determination of the quantitative importance of each of these 

 factors is a problem of great difficulty, but one of great economic impor- 

 tance and scientific interest. 



The effect of igneous rocks in the production of ore deposits is differ- 

 ent in the zone of fracture and the zone of flowage. In the zone of fracture, 

 as has been fully pointed out, the igneous rocks follow the openings toward 

 the surface, and when they finally reach the surface are spread out over it. 



In the zone of fracture the effects of igneous rocks in the belt of 

 weathering and in the belt of cementation need separate consideration. In 

 the belt of weathering, above the level of ground water, intruded rocks are 

 baked or calcined. They are steamed by the gaseous solutions. It is 

 possible that in this belt ore deposits may be segregated through the 

 influence of volcanism, but positive evidence that this is a fact is lacking. 



In the belt of cementation the igneous rocks are of great consequence. 

 There is here the direct effect of heating the intruded rocks, but far more 

 important than this is the indirect effect in heating the solutions. Where 

 the temperature rises above the critical temperature of water the solutions 

 are gaseous. It is, however, believed that the gaseous solutions are prob- 

 ably closely confined to the borders of the intrusive masses. The chief 

 effect is believed to be in reference to the aqueous solutions. It will be 



