374 ELEMENTARY GEOLOGY 
change, where the pressure was great, gradually be- 
comes less and less metamorphosed as we pass to por- 
tions of the mountains, where for one reason or another 
the pressure was lessened. 
Again, in the close neighborhood of a great mass 
of intruded granite, the rocks may be so highly altered 
that we cannot easily tell what they originally were 
Fia. 227. 
Photograph in a slate quarry, showing slaty cleavage crossing the beds of 
folded rock. 
(Plate 15), while as the distance from the source of 
heat increases, the former condition becomes more and 
more distimctly apparent, until finally the unchanged 
rock appears. 
Sources of the Heat and Pressure. —'The source of 
the water, which is so important an agent of meta- 
