226 STRUCTURAL GEOLOGY. 



Cause of Metamorpliism. 



There are two kinds of metamorphism which must be 

 distinguished, viz., local or contact metamorphism,, andre- 

 gional metamorphism. The former is produced by direct 

 contact with fused matter, as in dikes or intercalary beds 

 (page 217). There can be no doubt as to the cause in 

 this case. It is intense heat. But the effect of the heat 

 extends but a little way from the plane of contact. In 

 regional metamorphism, on the contrary, the change is 

 universal over hundreds of thousands of square miles and 

 thousands of feet of thickness. In these cases there is no 

 evidence of intense heat in every part ; the heat was prob- 

 ably very moderate. It is of this kind that we now wish 

 to explain the cause. 



The Agents of regional metamorphism are 1. Heat ; 

 2. Water ; 3. Alkali ; 4. Pressure ; 5. Crushing. 



To produce metamorphism by heat alone, i. e., dry heat, 

 would require a temperature of 2,500 to 3,000, but in the 

 presence of water a very moderate heat will change rocks. 

 At 400 Fahr. (= 205 0.), incipient change commences ; 

 and at 800 Fahr., complete hydrothermal fusion takes 

 place. If any alkaline carbonate be present in the water, 

 these effects occur at still lower temperature. The quan- 

 tity of water necessary is only ten to fifteen per cent. ; in 

 other words, the included water of sediments is amply 

 sufficient. Pressure is necessary, because it is impossible 

 to have even such moderate heat in the presence of water, 

 unless the whole be under pressure. 



Application. Suppose, then, we have sediments ac- 

 cumulating along a shore-line, or at the mouth of a river 

 until a thickness of 10,000, 20,000, or 40,000 feet is 

 reached. It is evident that the isogeotherms (interior 

 isotherms) would rise, and the lower portion of the sedi- 

 ments with their included waters would be invaded by 

 the interior heat of the earth (Fig. 134). At the rate of 



