METAMORPHISM 371 
there, has escaped complete change, so that from these 
we are able to discover the original condition. 
Position of Metamorphic Rocks. —It may be said 
that metamorphic rocks are found in the earth’s crust 
in three positions: (1) near some mass or bosse of in- 
truded igneous rock; (2) in mountains, deep below the 
original surface, and in the very core; (3) among the 
most ancient rocks, 
particularly the 
lowest and oldest 
of all, the Archean. 
The first are small 
in area, the second 
usually fall within =— 
relatively narrow == es = 
zones, and the third Se 
occur in great areas. 
Not many years 
ago it was believed Diagram showing flattening of pebbles by the 
pressure during metamcrphism. 
that all these meta- 
morphic rocks represented the most ancient of strata, 
and were a part of the original crust of the earth; 
but careful studies have shown that this is not so, and 
that many very much altered rocks belong to later ages, 
and are really transformations of sediments belonging 
to the same general age as deposits of shales, limestones, 
and sandstones in neighboring regions. 
