288 METAMORPHISM 



Metamorphism is of two quite distinct kinds: (i) contactor 

 local, and (2) regional metamorphism. 



I. Contact Metamorphism 



This is the change effected in surrounding rocks by igneous 

 intrusions, dikes, bosses, etc. The rock invaded and metamor- 

 phosed may be either sedimentary, igneous, or already metamor- 

 phic, and the effects may be very marked, or surprisingly small ; 

 indeed, it is often quite impossible to say why the changes should 

 be so insignificant. Plutonic rocks are more effective in producing 

 these changes, because they are, presumably, hotter and retain 

 their heat longer. Magmas which contain an abundance of the 

 mineralizing vapours (see p. 192) produce much more effect than 

 those with only a small quantity of such vapours. For this reason 

 acid magmas are more effective than basic. Much, too, depends 

 upon the nature of the invaded rock; sediments which contain 

 large percentages of alumina and lime are much more readily and 

 profoundly changed than those which are made up almost entirely 

 of silica. The distance to which the zone of change extends is 

 wider when the intrusive mass. cuts across the strata than when it 

 follows the bedding planes, so that a dike or boss is more effective 

 than a sheet. 



We may now consider some examples of contact metamorphism, 

 and, for this purpose, shall select only the changes of sedimentary 

 rocks; for those of the other classes require a treatment too minute 

 and refined for an elementary work. We may note, in passing, 

 however, that some of the veins given off from granite bosses, 

 which have invaded other igneous rocks, are probably of a meta- 

 morphic nature and due to the penetration of vapours. 



In a series of strata which have been invaded by an igneous 

 mass, we find a gradual change from the unmodified rock which 

 lies beyond the reach of the transforming agencies, to that at the 

 actual contact with the igneous mass. Along this line of contact 

 the strata are so thoroughly reconstructed that often only a micro- 

 scopical examination will distinguish the changed sediment from 



