34 ROCK-METAMORPHISM. 



CHAPTEE VI. 



WHY SOME METAMORPHIC ROCKS HAVE BEEN 

 MINERALIZED AND OTHERS METHYLOSED. 



WE adopt the general opinion that all stratified metamorphics 

 have been in the first instance aqueous sediments, and that 

 heated water has been concerned in developing their present 

 features. Moreover we assume, though without knowing 

 whether others are of the same opinion, that, before their meta- 

 morphism took place, these aqueous sediments retained more 

 or less of the water they originally contained, and that, on 

 their becoming buried at great depths, where necessarily an 

 elevated temperature and other favourable conditions prevailed, 

 this original water played a part in mineralizing them*. 



Adhering to the foregoing as postulates, and limiting our- 

 selves to a well-known case in point, we offer it as our opinion 

 that the Archaean argillytes, sandstones, &c. became minera- 

 lized, when at great depths, by means of the water they were 

 originally charged with, and in consequence of the high tem- 

 perature and great pressure under which they were placed. 



As the water in such cases must have extended over vast areas^ 

 its action has necessarily been on a regional scale. 



Although, on our view, mineralized metamorphism has been 

 effected to an important extent by the intervention of water, it 

 must be admitted that the evidences are not very abundant; 

 for the resulting minerals are wholly anhydrous. The strongest 

 evidence consists in the presence of liquid bubbles (well known 

 from the researches of Sorby, Zirkel, Allport, and others, as 

 occupying cavities) in the quartz, feldspar, and other minerals 

 characteristic of the mineralized metamorphics f. 



* It was to be expected that Scheerer, who advocates the aqueous origin 

 of granite, would make this rock to contain its " primitive water." 

 t Natrolite, talc, chlorite, and other hydrous minerals in granite, accord- 



