298 METAMORPHIC ROCKS 



blende schists occur as belts or bosses in metamorphic areas and 

 are largely developed around Lake Superior. 



The schists already described are much the most abundant 

 varieties of the group, but there are numbers of others. Thus, we 

 have talc and chlorite schists, both of which are due to alteration, 

 principally of hornblende schist, and graphite schist, which has 

 quantities of that carbonaceous mineral along its foliation planes. 



Summary. — Structural geology brings vividly before us the 

 innumerable changes through which the earth's surface has passed 

 and which are recorded in the rocks. The sedimentary rocks, 

 originally laid down under water in approximately horizontal posi- 

 tions, have been upheaved into land surfaces, either without losing 

 that horizontality, or being tilted, folded, compressed, or even 

 violently overturned. Or, they may be fractured and dislocated in 

 great faults and thrusts. These movements we have found to be 

 due to enormous lateral compression set up within the crust of 

 the earth, a compression probably generated by the shrinkage of 

 the cooling globe. Whether folding or faulting shall result from a 

 given compression depends upon the rigidity of the strata, upon 

 the load which overlies them, and the sudden or gradual way in 

 which compression is applied: The results of compression on a 

 large scale are accompanied by certain minor changes not less 

 characteristic. Compressed rocks are cleaved, fissile, or schistose, 

 according to the intensity of the action, and whether the rocks 

 affected are in the zone of flowage or of fracture. These changes 

 may go so far as to completely reconstruct the minerals of the 

 rocks, destroying the old, generating new, and obliterating the 

 original character of the strata. Thus, displacements, dislocations, 

 cleavage, fissility, and dynamic metamorphism are but the varying 

 results of lateral compression, acting under different conditions. 



Another class of rocks — the igneous, massive, or unstratified — 

 we found to have penetrated and overflowed the strata, and to have 

 consolidated in the fissures and cavities which they have made for 

 themselves, or to have been poured out freely on the surface. 

 According to the circumstances under which these masses have 

 cooled, the resulting rock is of glassy, porphyritic, finely or coarsely 



