170 STUDIES FOR STUDENTS 



Volcanic subaerial breccia. — Volcanic breccias laid in open air 

 include: 



i. Flow breccia, in which unrounded fragments have become 

 incorporated in flowing lava, either from its frozen and broken 

 crust or from material, volcanic, residual, or of other origin, over- 

 ridden by the advancing stream. 



2. Tuff breccia, made up of the f ragmen tal products of explo- 

 sive eruptions. The matrix consists of the finer materials of the 

 eruption and in some instances has been washed in by mud-flows. 

 Or the matrix may be formed by gangue and ore stuffs deposited 

 in the interstices by heated waters. Tuff breccias often include 

 fragments of the country rock torn from the sides of the duct below 

 the base of the volcanic cone. 



Subaqueous breccias. — Breccias accumulated under water may 

 be divided into three general classes with the following sub- 

 divisions : 



i. Breccias of subaerial fragmentation 



a) and b) Subaqueous talus and landslide breccia 



c) Raft breccia, deposits from rafts of ice (iceberg or ice floe), trees, or 

 seaweed. 



d) Desiccation breccia 



e) Subaqueous volcanic breccia 



2. Breccias whose fragmentation is the work of aqueous agencies or of agencies 

 working in water 



a) Shoal breccia 



b) Reef breccia 



c) Beach breccia 



d) Tide-glacier and shore-ice breccias 



3. Breccia whose fragmentation is due to internal stresses 



a) Glide breccia due to overload, earthquakes, deformation, undercut 



Talus and landslide subaqueous breccias. — -These two varieties 

 may be taken up together because of certain common features. 

 Both consist of local beds of angular fragments from near-by 

 sources, intercalated between younger sedimentary strata. Suit- 

 able topographic conditions for the formation of each are found in 

 the fjords and rias of a rugged coast and in mountain lakes, espe- 

 cially Chelans, lying in oversteepened glacier troughs. The matrix 

 is partly of the same material as the fragments and partly of infil- 



