292 Barrel! — Physical Effects of Contact Metamorphism. 



a quarter to half a mile from the intrusions and in a minor 

 degree extends much farther. 



The final test of shrinkage and absence of infiltration would 

 be to measure a section of these rocks and to examine their 

 chemical composition beyond the limits of the district where 

 free from metamorphism, and to compare these results with 

 similar data derived from the metamorphic zone. To do this 

 with the necessary accuracy, however, would have required 

 much time, and the limits of the field season did not permit of 

 the work being undertaken. 



From all the evidence at hand it does not appear that except 

 in local and special instances any infiltration has occurred. 

 By referring to the table of metamorphic minerals previously 

 given, it is found that the minerals of the Starmount formation 

 in their observed proportions would originate from a sediment 

 originally consisting of : 



Quartz 21-3 



Kaolinite 25*0 



Calcite 46'6 



Magnesite 7*1 combined with calcite as dolomite. 



100-0 



In the process of metamorphism this mass of strata has 

 lost approximately 28 per cent of its weight and 45 per cent 

 of its volume, from 70 to 90 times its volume of water vapor 

 and 320 volumes of carbon dioxide, the gases being measured 

 at 0° C. and 760 mm . 



The prevention of infiltration. — The causes preventing the 

 addition of solid material are doubtless to be found in the fact 

 that the gases must escape and that while this is taking place 

 recrystallization is going forward ; so that by the time relief is 

 obtained from internal pressures and heated waters would be 

 able to make their way inward, the rock is already dense and 

 crystalline. To realize the degree of pressure exerted by the 

 escaping gases it is necessary to state more exactly the condi- 

 tions obtaining. The individual igneous intrusions may be 

 presumed to break through the strata with considerable 

 rapidity and at temperatures of 1200° C. to 1500° C. But 

 metamorphism proceeds readily at a temperature of 180° C. 

 and for higher temperatures it would take place with still 

 greater rapidity. The changes of dehydration and decarbona- 

 tion are those requiring the absorption of heat, while the 

 ultimate contraction of the sediments is a heat-producing 

 operation. The final expansion of the carbon dioxide would 

 absorb heat, but if it was able to occur only at. a distance from 

 the place of decarbonation the absorption of heat would not 



