METAMORPHISM 369 
carrier, water, has been heated. One of the best illus- 
trations of this water action in moving materials during 
metamorphism, is found in the quartzites, which are 
really altered sandstones firmly cemented by quartz, so 
that the rock looks like a solid mass of quartz. Some- 
times the deposited quartz has gathered, in the form 
of true crystals, about the grains of sandstone, each 
grain serving as centre to a grow- 
ing crystal (Fig. 224). 
During metamorphism there are 
often indications of high pressure, 
—now in the crumpling of folded 
layers, as paper may be crumpled 
(Figs. 225 and 228), again in the 
squeezing out of pebbles, even 
those of quartz, into elliptical forms 
Fic. 224. 
(Fig. 226), or more rarely, into Diagram to illustrate growth 
of a quartz crystal (C) 
sheet-like layers. Among these ¢ ov ea Saale (5Y. 
signs of pressure, there is some- 
times evidence of flowing, which indicates the presence 
of heat. Furthermore, rocks are not infrequently 
crushed, and the minerals broken as a result of great 
pressure. ‘This crushing is also accompanied by move- 
ment, producing a banded structure in the rock thus 
metamorphosed. 
In all these cases, there comes about an arrangement 
of layers, which may not lie parallel to the original 
2B 
