DEFORM A riON OF ROCKS 6o I 



obtaining between schistosity and bedding. Readjustment 

 between the beds, as explained on a previous page, may also 

 have assisted in the process. 



In a third case, a great boss of intrusive igneous rock may 

 cause the secondary structure to be everywhere parallel to it, 

 without reference to the direction of previous structures. This 

 case probably differs but little from that of direct thrust, but the 

 direction of thrust gradually varies through 360° in circumscrib- 

 ing the mass, being at all times radial. The material pushed 

 aside obe_ys at each point the law of normal plastic flow, just as 

 in ordinary orographic movements. The new minerals develop 

 with their shortest diameters in the direction of thrust. Old 

 minerals are mashed into similar forms in parallel positions. 

 The heat of the igneous rocks furnishes hot solutions which help 

 to transform the old minerals. As the direction of the thrust 

 varies gradually around the intrusive, the secondary structures 

 follow, and therefore form a zone around the intrusive, the 

 layers of which may be compared to those of an onion (see pp. 

 454-456). 



The process may be complete, and old structures, such as 

 bedding, previous cleavage, or previous fissility, may be wholly 

 destroyed. In other cases traces of these structures may still be 

 found. Where the earlier structures are wholly obliterated near 

 the intrusive, they may appear gradually in passing away from 

 it. Thus in the same rock mass several structures may occur. 

 The mica-schists about the intrusive granite core of the Black 

 Hills are an excellent illustration of this case. 



MODIFICATIONS OF SECONDARY STRUCTURES. 



The partings of fissility may be concentrated here and sparse 

 there. In the cracks between the laminae a new mineral or 

 minerals may be deposited from water solutions, and the second- 

 ary zones of greatest fissility would then have a composition 

 different from that of those w'hich are less fissile. Moreover, 

 the parted laminre and the minute layers of infiltrated material 

 mav be of different composition. This would give a minute 



