DEFORM A TION OF ROCKS 



627 



form. A basal conglomerate, on the other hand, if followed 

 along the strike and dip, may change its character, but it will 

 be a gradual change into the ordinary mechanical sediments, 

 whereas an autoclastic rock is likely to have very sudden varia- 

 tions in character. 



Fui. 17. — Chert breccia, an autoclastic rocl-c resting upon truncated minor folds 

 of limestone. 



Using all of the above criteria, it is difificult in some cases to 

 discriminate between an autoclastic rock and a true conglomer- 

 ate. Usuallv, however, if an area be studied sufficiently long, 

 and if the relations be examined closely a true judgment may 

 be reached. 



Autoclastic rocks are not likely to develop from shales and 

 limestones, but if near enough to the surface even these rocks 

 may become brecciated. As a consequence of orogenic move- 

 ments, in the zone of combined fracture and flowage, where the 

 alternate layers are thick, the shales and limestones may flow 

 and the cherts and quartzites become brecciated. The brecciated 

 and nonbrecciated layers, under these circumstances, may not 

 become mingled to any considerable degree. Thus we may 

 have a set of autoclastic rocks interstratified with layers which 

 show no sign of brecciation. It may be that the plastic layers, 

 as a result of the stress, may be minutely corrugated. The 

 movement of the broken particles in the rigid layers against the 

 crests and troughs of the folds may have truncated them. We 

 might then have pseudo-conglomerates resting upon folded 

 truncated lavers (Fig. 17), and it might be concluded that there 



