884 A TREATISE ON METAMOEPHISM. 



The original quartz is granulated and recrystallized, as in the other 

 varieties of sands. As the feldspars recrystallize, the micas are likely to 

 form abundantly, especially biotite, since iron and magnesium are plentiful. 

 (PI. X, B.) But while the feldspar may in part pass into other minerals, 

 it in large part recrystallizes as feldspar, with perhaps a change in species. 

 Such new feldspar is apt to be an abundant constituent in the recrystallized 

 rocks. Frequently the ferromagnesian, minerals are so plentiful that the 

 amphiboles develop extensively. In the abundance of amphibole these 

 rocks differ from the schists and gneisses of the quartz-sand and quartz- 

 feldspar-sand families. The particular amphibole which develops in a given 

 formation depends, of course, upon the composition of the rock, but actino- 

 lite and common hornblende are very abundant, (PI. X, C and D.) Rarely, 

 where the metamorphism is of a very extreme nature, the pyroxenes may 

 also develop to some extent, but any considerable quantity of these min- 

 erals is rather unusual. It is probable that the greater abundance of 

 amphibole is due, in part at least, to the fact that in this group of min- 

 erals the ratio between the calcium and the magnesium is 1:1, whereas 

 in the pyroxenes it is 3 : 1. So far as decomposition has taken place in 

 the original sands, the calcium is likely to have been extracted in greater 

 proportion than the magnesium (see pp. 515-516); therefore many of the 

 ferromagnesian sands may have been so depleted in calcium that pyroxenes 

 can not readily form. Associated with the more abundant minerals, the 

 heavy metamorphic minerals, such as garnet, staurolite, and tourmaline, 

 may appear. However, these minerals are not likely to form so abundantly 

 as in the pelite family, and the aluminum-silicate group of minerals, so com- 

 mon in the pelites, is rarely found. So far as hydrous minerals have been 

 produced in the belt of cementation, they are decomposed. The material 

 of the chlorites may largely go into the micas, amphiboles, garnets, stauro- 

 lite, and other minerals. The zeolites may pass into the feldspars, especially 

 into albite or oligoclase. In short, any of the combinations of hydrous 

 minerals which have been formed by the alteration of a heavy original min- 

 eral may recombine to reproduce the original heavy mineral; for instance, 

 serpentine, gibbsite, and kaolin may unite to produce biotite; kaolin and 

 quartz, or gibbsite and quartz, with sodium-bearing minerals, may unite to 

 produce albite; serpentine and quartz to produce enstatite, etc. 



