Studies for Students 



A CLASSIFICATION OF BRECCIAS 



W. H. NORTON 



Cornell College, Iowa 



Few geologic structures so lend themselves to diverse inter- 

 pretations as the beds of broken rock called breccia. Example 

 after example might be cited of breccias of Europe and America 

 which have been differently explained by different students during 

 the last half-century and as to whose origin no consensus is yet 

 attained. This diversity of opinion seems partly due to the large 

 number of processes by which rocks are broken up, assembled, 

 and cemented into breccia, and to the fact that breccias may offer 

 no very obvious and indisputable evidences of the method of their 

 making. Diagnosis generally requires the use of multiple working 

 hypotheses and may proceed chiefly by the process of elimination. 

 For this reason a genetic classification which the writer has pre- 

 pared in connection with a field study of certain breccias affecting 

 the Wapsipinicon stage of the Devonian of Iowa may prove of 

 interest to students of these structures. 



The diagnosis of a breccia requires the close observation of its 

 most intimate characteristics as well as of its associations with the 

 adjacent rocks. The matrix may be like or unlike the fragments 

 lithologically. It may be a chemical precipitate, a sedimentary 

 deposit, or the detritus of attrition. In volume it may be greater 

 or less than the fragments — interstitial, merely filling the spaces 

 between the fragments closely packed, or preponderant, forming 

 the larger part of the rock-mass in which the fragments are sporadic. 



The fragments may be of any size, from huge blocks down 

 to chipstone. Lithologically they may be similar or dissimilar, 

 according as they result from the fragmentation of a homogeneous 

 rock-mass or from that of heterogeneous beds. They may be 

 sharply angular, more or less rounded by attrition in earth move- 

 ments, or even in part water- worn and approaching a conglomerate. 



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