GREYWACKE 293 



IV. Metamorphic Rocks 



A. NON-FOLIATED ROCKS 



These represent the less advanced stages of metamorphism, in 

 which the forces of compression may have produced cleavage or 

 fissility, but not foliation. The more important rocks of this 

 class are of sedimentary origin, and it will be unnecessary for us 

 to consider the igneous rocks which have been changed, though 

 not to the extent of producing foliation. 



Quartzite is derived from the metamorphosis of sandstone, and 

 between the two kinds of rock are found such complete transitions, 

 that the separation of them seems almost arbitrary. In a typical 

 quartzite the rock is crystalline, and the quartz deposited around the 

 sand-grains is in crystalline continuity with those grains, though 

 the microscope still reveals the original fragmental nature of the 

 rock. Quartzites also result from the metamorphism of conglom- 

 erates, and the pebbles are sometimes much flattened by compres- 

 sion. If the sandstone or conglomerate contained impurities, other 

 minerals beside quartz are generated ; if any considerable quantity 

 of clay was present, mica will be produced and, it may be, in such 

 abundance that the rock passes into mica schist (see below). 



Quartzites are formed both in contact and regional metamor- 

 phism, but the change is principally due to cementation, large 

 amounts of silica (estimated as one-sixth of the original quantity 

 present in the sandstone) being brought in and deposited from 

 solution. Many quartzites do not appear to have been subjected to 

 great compression, though- others are cleaved or fissile (Fig. 113). 



Greywacke is a hard, crystalline rock, of banded appearance 

 and characteristic grey colour. The sedimentary original is either 

 a mudstone, an arkose, or a mass of fragments of felspar, mica, 

 quartz, and other igneous minerals which have been mechanically 

 abraded, but little or not at all decomposed. The change is due 

 largely to cementation, the sand-grains being enlarged as in quart- 

 zites, and silica is deposited in the interstices, binding the whole 

 mass firmly together. The other minerals undergo a great variety 



