372 ELEMENTARY GEOLOGY 
For instance, this is true of a good part of the rocks of New 
England, which were all once supposed to belong to the very oldest 
epoch of the earth’s history. Of the truly ancient rocks of these 
early ages, the Highlands of New Jersey and the Adirondacks 
of New York furnish the best known instances in eastern United © 
States, though parts of New England are undoubtedly of the 
same period. 
Those metamorphic rocks that are situated in mountain cores, 
are in places where there has been great pressure and movement, 
and probably much heat. The same is true of the metamorphic 
rocks found near igneous intrusions. Perhaps the most ancient 
Archean strata really represent the original crust of the earth, 
or else deposits made and changed at a time when there was much 
higher heat in the surface rocks than we find at present. Of this, 
however, there is no definite proof; and in the absence of data, 
we can only speculate. 
Causes for the Changes. —Since there are rocks 
whose changes can be seen and studied, it is well to 
content ourselves for the present with a consideration 
of these, without assuming that the same variations 
have occurred in the metamorphic rocks of more 
ancient date. Any constant application of high heat, 
especially if accompanied by the work of water (which 
is always present in the rocks), will cause the minerals 
to change. Under sufficient heat, the rocks will melt, 
though in real metamorphic beds there is no sign of 
actual melting. 
If allowed to work long enough, these changes will 
cause a recrystallization of the elements. Perhaps the 
water will take quartz into solution in one place, 
