METAMORPHISM OF ROCK-MASSES. $77 



acid is entirely expelled only from very fine-grained mixtures 

 of calcareous and ashy materials : approaching the contact, the 

 complete decomposition of the calcite is found to -extend to suc- 

 cessively coarser-grained rocks. Another line of inquiry is 

 offered by the texture of the metamorphosed rocks themselves, 

 of whatever lithological nature, in a district of metamorphism 

 surrounding a large igneous intrusion. The size of the individual 

 crystals of secondary minerals increases towards the contact with 

 the intrusive rock : this may be taken to indicate that the migra- 

 tion of material within the mass of a rock undergoing meta- 

 morphism has more latitude when the temperature is higher. 

 For various reasons, however, it would be unsafe to found numeri- 

 cal results upon such observations. The crystals of certain meta- 

 morphic minerals attain to considerable dimensions by virtue of 

 their power of enclosing a large amount of foreign material ; 

 others, again, can apparently push aside solid impurities to make 

 room for their own growth. The texture of the metamorphic 

 rocks examined is still, however, in general accord with the con- 

 clusions reached by other methods of inquiry. 



The question naturally arises whether the limit of migration 

 of material is the same for different substances. On this point 

 we have but little information. Among the various types of 

 "spotted" rocks described in aureoles of metamorphism is one in 

 which the spots are simply spaces free from the secondary brown 

 mica abundant in the general mass of the metamorphosed rock. 

 Since the iron compounds in the rock must originally have had 

 a generally uniform distribution, the phenomena of the spots 

 indicate a movement of ferrous oxide, and the radius of the spots 

 gives a measure of the extreme limit of such movement. In the 

 cases examined this is about one-twentieth of an inch, and we 

 may infer that the greatest distance of migration of ferrous oxide 

 has been about the same as that of silica at a similar tempera- 

 ture. 



Not to insist unduly upon precise estimates, these and similar 

 observations certainly tend to show that in thermal meta- 

 morphism no interchange of material takes place except between 



