METAMORPIIIC ROCKS. 



227 



100 increase per mile (page 132), the lower portion of 

 the sediments 20,000 feet thick would be 400 -f 60 

 (mean surface temperature) = 460, and 40,000 feet of the 

 sediments would be at the bottom 860. Now, we actually 



FIG. 134. sb, original sea-bottom ; s'b', sea-bottom after sediments, set, have accu- 

 mulated ; , isogeotherms of 800 and 400 ; , same after accumula- 

 tion of sediments. 



have strata 20,000 and 40,000 and even more feet thick. 

 The lower portions of such strata must be completely 

 metamorphic. The figure (Fig. 134) shows how the pro- 

 cess takes place. 



Crushing. Pressure alone is a condition, but not a 

 cause of heat. But pressure producing motion, or crush- 

 ing, crumpling, is an active cause of heat. Now, we 

 usually find metamorphism associated with most complex 

 crumpling of strata. The heat must have been increased 

 also by this cause. 



Even igneous rocks, by pressure, mashing, and shear- 

 ing, may be made to assume the appearance of metamor- 

 phic stratified. Many schists, especially gneisses, are 

 formed in this way. 



