AUTOCLASTIC ROCKS 529 



Tlie following criteria are given by Van Hise as aids in dis- 

 criminating between autorudytes and true conglomerates (hydro- 

 rudytes) : 



1. "An autoclastic rock must derive its material mainly from 

 the adjacent formation . . . from the superior formation as 

 well as from the inferior. ... In true basal conglomerates, on 

 the other hand, while the material is very frequently derived in 

 large measure from the immediately subjacent formations, they 

 also usually contain a small proportion of material from various 

 foreign sources, and . . . not . . . from the overlying for- 

 mations, as may the autoclastic rocks." 



2. "In an autoclastic rock, . . . the pebbles . . . will 

 in many cases be found to be less rounded than in a true basal con- 

 glomerate. If . . . followed for some distance a considerable 

 variation will frequently be found — fragments being here well 

 rounded and there very imperfectly rounded. The well-rounded 

 fragments are concentrated, as are also the angular fragments. A 

 basal conglomerate, on the other hand, has a considerable uniform- 

 ity in the degree of the rounding of ifs pebbles in passing along 

 the same horizon, but at the same place the large fragments may be 

 angular and the small ones well rounded . . ." 



3. "In many cases the interstices of an autoclastic rock are 

 filled with material of a vein-like character, whereas in a basal 

 conglomerate the filling material is largely fine detritus. But some- 

 times . . . the filling material of an autoclastic rock may be 

 water-worn grains of sand, which have been separated by dynamic 

 action, and are therefore indistinguishable from the ordinary matrix 

 of a true conglomerate." 



4. "In most instances a bed of autoclastic rock, if followed, 

 may be traced into on ordinary brecciated or partly brecciated 

 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 auto- 

 clastic rock is likely to have very sudden variations in character." 

 (Van Hise-3o:d6'o-<^/.) 



Intraformational Breccias. 



These are contemporaneous phenomena, formed as one of the 

 sequential divisions of a single rock series. They are almost wholly 

 confined to calcareous rocks, and seem to be most typical of calci- 

 lutytes or fine calcarenytes. Such rocks, composed largely of the 



