DEFORMATION OF ROCKS 625 



from a true conglomerate. Fortunately, in most cases it is 

 possible to find transition phases between such a rock and one 

 in which the process has not gone so far, and thus one is 

 enabled to determine that the rock is autoclastic. 



ZONE OF AUTOCLASTIC ROCKS. 



The zone in which autoclastic rocks ma_y be produced is con- 

 fined to the outer 10,000 meters of the earth's crust, and the 

 formation of widespread autoclastic rock is probabl}- limited to 

 the outer 5000 meters. At a depth greater than the larger 

 number the pressure in all directions exceeds the crushing 

 strength of any rock, and therefore if it were possible for 

 crevices to form such as are necessary to produce brecciation 

 they would be almost immediately closed by flowage. Conse- 

 quently, at great depths it is to be supposed that no crevices 

 form in the rocks as the result of dynamic movements, and 

 therefore that no breccias are produced. 



From the foregoing it follows that autoclastic rocks may 

 develop whether the formations concerned are homogeneous 

 or heterogeneous. Also that they may develop whether the 

 beds are all within the zone of fracture for them or whether 

 they are in the zone of fracture for a part of them, and in the 

 zone of flowage for the other part. In the first case dynamic 

 breccias are likely to form. In the second case only the stronger 

 rocks are broken, the fragments being buried in the members 

 which flowed, and pseudo-conglomerates are frequently formed. 



RELATIONS OF AUTOCLASTIC ROCKS TO BASAL CONGLOMERATES. 



Since it is possible that pseudo-conglomerates may be mis- 

 taken for true basal conglomerates, the criteria which discrim- 

 inate the two are of great importance. 



( I ) An autoclastic rock must derive its material mainly 

 from the adjacent formations. If, for instance, it is produced 

 from intcrstratified layers of limestone and quartzite, it will con- 

 tain only limestone and quartzite detritus, and the fragments 

 will be mainly from the more brittle formation. Further, an 



