626 STUDIES FOR STUDENTS 



autoclastic rock may have a part of its material from the superior 

 formation as well as from the inferior. However, in some cases 

 the brecciated layer may itself have been conglomeratic, 

 although not a basal conglomerate, and thus some material from 

 extraneous sources will be found. But in most instances the 

 material is of local origin. 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 do not contain any material from the over- 

 lying formations, as may the autoclastic rocks. 



(2) In an autoclastic rock, if the pebbles are closely 

 examined they will in many cases be found to be less rounded 

 than in a true basal conglomerate. If the belts of brecciation. 

 be followed for some distance a considerable variation will fre- 

 quently be found in this respect, 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 

 uniformity in the degree of the rounding of its pebbles in pass- 

 ing along the same horizon, but at the same place the large frag- 

 ments may be angular and the small ones well rounded. In a 

 basal conglomerate very near to the underlying formation many 

 of the contained fragments may be angular, but in an extreme 

 case the fragments of a basal conglomerate are upon the aver- 

 age usually not so angular as those of an autoclastic rock. 



(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 finer detritus. But 

 sometimes, as in the case mentioned of a semi-indurated quart- 

 zite, 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 an ordinary brecciated or partly brecciated 



