1 62 STUDIES FOR STUDENTS 



of breccia the matrix is of the same age as the fragments. In 

 other classes it is younger, and in still others it may even be supplied 

 by an older bed. 



Breccia may be of slight extent and evidently due to local 

 causes. On the other hand, some of the most perplexing breccias 

 are regional and demand causes equally widespread in operation. 



According to the conditions under which they accumulate, 

 breccias may be classified as subaerial, subaqueous, or endolithic , 

 formed within the lithosphere, the earth's crust. 



In classifying breccias genetically it will be remembered that 

 the making of a breccia involves, or may involve, three distinct 

 processes — fragmentation, assemblage of the fragments, and 

 cementation by the introduction of the matrix. In any given 

 breccia all of these processes may belong to the same or to different 

 categories. The genetic classification may have in view either 

 fragmentation, as in crush breccias, or assemblage, as in several 

 other types. 



Subaerial breccias, in which both fragmentation and assemblage 

 are above ground, may be classified as follows: 



Residual breccia Bajada breccia 



Talus breccia Glacial breccia 



Rock-glacier breccia Volcanic breccia 



Landslide breccia 



Residual breccia. — This type is formed of the angular debris of 

 the waste mantle. It has been designated as "basal breccia," 1 

 since it corresponds in position to a basal conglomerate. But all 

 subaerial breccias covered and preserved by the deposits of a 

 transgressing sea may correspond equally well to basal conglom- 

 erates, so that some designation seems preferable which suggests 

 the residual origin of this specific kind. • 



Residual breccias develop especially on Karst topographies 

 where the limestone of the country rock contains cherty beds. 

 Under long denudation the surface with its characteristic sink- 

 holes and closed valleys comes to be covered in places to some depth 

 with breccia of sharp-edged chips of flint, and this by submergence 

 may be incorporated into the sequence of the geologic formations. 



1 W. S. Smith and C. E. Siebenthal, U.S. Geol. Surv., Geol. Atlas, Joplin Folio 

 (148), p. 9. 



