290 METAMORPHISM 



injection is not a very sharply marked one, because superheated 

 water and molten magmas appear to mix in all proportions. The 

 difference between the two processes seems thus to be largely a 

 question of the quantity of water present. 



Contact metamorphism, as its name implies, is a local phenom- 

 enon, but a widely ramifying and complex system of igneous 

 intrusions may change large areas of sedimentary rocks. 



II. Regional Metamorphism 



This term applies to the reconstruction of rocks upon a great 

 scale, in areas covering, it may be, thousands of square miles, and 

 evidently other processes in addition to those of contact meta- 

 morphism are needed to explain such wide-spread changes. A 

 very general characteristic of such metamorphic rocks is foliation, 

 or schistosity. This is either cleavage or fissility (see p. 260), or 

 both, which causes the rock to part into plates with rough or 

 undulating surfaces, due to the presence of flakes of some mineral 

 arranged in rudely parallel planes. Schistosity is connected by 

 every transition with cleavage or fissility, and represents, an ad- 

 vanced degree of metamorphism, as the latter processes are 

 incipient stages of the same. 



The first step in metamorphism consists in a mere hardening 

 of the rock, accompanied with the loss of water and other vola- 

 tile substances. In the second stage the component minerals 

 are crystallized, but new compounds are sparingly formed. The 

 shearing or crushing to which the mass has been subjected fre- 

 quently change minerals into paramorphic forms, i.e. those which 

 have the same chemical composition, but different crystalline 

 form and physical properties. For example, aragonite is thus 

 changed to calcite, and augite to hornblende. This stage is fre- 

 quently accompanied by cleavage or schistosity. In the more 

 advanced stages the rocks are foliated, and complete chemical 

 reorganization may take place, with the abundant development 

 of new minerals. The compression and consequent shearing and 

 crushing to which the rocks have been subjected are the princi- 



