14 
HAROLD’S DISCUSSIONS. 
change. Scarcely are they formed when forces 
begin to alter them. These forces are heat, water, 
and pressure. Sedimentary rocks are often so 
changed by these forces that all signs of stratification 
are obliterated, and the character of the rocks be¬ 
comes more or less volcanic. Such rocks are known 
as metamorphic {jneta, after, and morphe^ form); we 
say that they have been metamorphosed. The changes 
Fig. 8.—Specimen of crumpled gneiss. Photograph by 
G. H. Williams. 
which thus take place are numerous and often com¬ 
plex, depending upon the relative amount of heat, 
water, and pressure present. Only a few of the more 
evident changes can here be mentioned. 
Clay rock, which is fine-grained, can easily be 
detected by moistening a little and breathing upon 
it, for it then has a peculiar clay odor. If a stratum 
of this rock be pressed into a layered condition, it 
