HEAVY MINERALS FORMED DEEP IN LITHOSPHERE. 685 



such minerals as garnet, staurolite, and other heavy minerals form by the 

 metamorphosis of many of the igneous rocks without any necessary change 

 in average chemical composition. 



Not only may such changes as those given occur, but there may be 

 repeated changes. These are especially likely to occur with changes in 

 the pressure conditions. Under the law that the greater the pressure the 

 heavier the minerals formed, there maybe repeated recrystallizations of the 

 rocks. Minerals produced at an early stage under conditions of moderate 

 temperature and pressure are destroyed and minerals of higher specific 

 gravity are produced. Thus a mud may change to shale, thence to slate, 

 thence to mica-slate, thence to andalusitic micaceous schist, thence to gar- 

 netiferous, staurolitic, and C3^anitic micaceous schist or gneiss. In propor- 

 tion as the pressure is great and the temperature high, the tendency is to 

 produce heavier and heavier minerals. Thus a rock which had become 

 more and more deeply buried may be recrystallized, or partly so, a number 

 of times, minerals of higher and higher specific gravity successively 

 appearing. It is entirely possible that in the deeper part of the lithosphere 

 and within the centrosphere unknown minerals are produced which are 

 heavier than any formed in the part of the lithosphere which has reached 

 the surface as the result of denudation. In this connection it is noteworthy 

 that the majority of the heavy silicate minerals developed in the zone of 

 anamorphism are those which, so far as they have been artificial^ produced, 

 have been formed either under igneous conditions or, if water were present, 

 under conditions of very high pressure and temperature. The latter is 

 notably the case for amphibole, pyroxene, quartz, and adularia, which 

 were obtained by Chrustschoff from aqueous solutions heated to a tempera- 

 ture of 550° G. a 



Metasomatism in the zone of anamorphism may take place under 

 mass-mechanical or mass-static conditions. The modifications under these 

 two sets of conditions are so different that it will be necessary to consider 

 them separately. 



ALTERATIONS IN CONNECTION WITH MASS-MECHANICAL ACTION. 



It has been seen that where deformation in the zone of anamorphism 

 is accomplished by rupture alone the result is ever to subdivide the rock 

 particles. (See pp. 673-675.) It was there indicated that under other 



Chrustschoff , K. von, Ueber kiinstliche Hornblende: Neues Jahrbuch, vol. 2, 1891, pp. 86-90. 



