1014 A TREATISE ON METAMORPHISM. ~ 



gated, and therefore help to develop the ores. Thus, in this zone, deforma- 

 tion and chemical reactions both favor the concentration of ores. In the 

 zone of anamorphism the chemical reactions absorb heat, and to that extent 

 work against the effect of deformation. 



In the western portion of the United States there have been recent 

 erogenic movements and also recent development of ore deposits. But 

 correlative with orogenic movements is volcanism, and it is pointed out in the 

 following section that volcanism also, for various reasons, is favorable to the 

 segregation of ores. It is therefore exceedingly difficult in any given region 

 to determine the relative quantitative importance of orogenic movements 

 and volcanism in the development of the ores. Until this study is seriously 

 taken up for extensive regions the relative importance, of the two must 

 remain largely a matter of speculation. 



VOLCAlSriSM. 



The great importance of volcanism in reference to metamorphism has 

 been fully dealt with in the previous chapters. The vast extent and great 

 thickness of the Tertiary volcanic rocks afford evidence of the prodigious 

 quantities of material which may be extruded in a single geologic period. 

 It has been explained that correlative with and below these extrusions 

 probably occurred intrusions on an even vaster scale. It has also been seen 

 that important volcanism and extensive orogenic movements are usually 

 simultaneous in the same regions. The relation between the two is that 

 of cause-effect. Orogenic movements produce numerous openings, which 

 extend deeper than those that exist under quiescent conditions. These are 

 taken advantage of by the intrusive rocks which are being forced toward 

 the surface. The enormous hydrostatic pressure of the upwelling lava is 

 also one of the factors in further fracturing. Consequently there is action 

 and reaction between orogenic movements and volcanism, each advancing 

 the other. 



Volcanism may promote the development of ore deposits in various 

 ways. 



First. The igneous rocks may furnish metals for the ores. In the case 

 of a given ore deposit the metal may be almost wholly derived from the 

 recent igneous rocks, almost wholly from earlier rocks surrounding the 

 later intrusion, or partly from the two sources. Illustrations will be given 

 later. 



