Handbook of Paleontology 33 



mentary series and may be important in marking divisions 

 of geologic time. 



Breccias are of a similar nature to conglomerates, but 

 the fragments corresponding to the pebbles of the latter 

 are angular and show little or no evidence of water action. 

 If they have been deposited by water at all there has been 

 very little transportation and they must be close to their 

 place of origin. As with the conglomerates the prevail- 

 ing constituent gives its name to the rock, and we find 

 quartzite breccias, limestone breccias etc. Breccias pro- 

 duced by the accumulation of fragmental material 

 through the eruptive activity of volcanoes are known as 

 volcanic breccias or volcanic agglomerates. Talus brec- 

 cias are derived from talus accumulations and fault 

 breccias from the accumulation of shattered rock frag- 

 ments along a fault zone (page 30). Glacial breccia may 

 be derived from glacial till, the fragments of which are 

 generally scratched and polished. Such consolidated tills 

 form a rock known as tillite. Arkosic sandstones (page 

 35) grade into arkose breccias. 



Sandstones are clastic rocks composed of grains of a 

 prevailing size between 2.5 mm and 0.05 mm. The grains 

 may be the size of peas and the sandstones then grade 

 into conglomerates ; at the other extreme the finer-grained 

 rocks grade into shales. Typical sandstones are those 

 composed of quartz sand, but all rocks of an arenaceous 

 (sandy) texture are included in this group. The grains 

 of sandstone tend to be spheroidal and the larger they 

 are the more perfect the rounding is apt to be. There is 

 a very great difference among sandstones as regards the 

 cementing material that binds the grains together. The 

 cement may be silica, a carbonate (calcite, dolomite or 

 siderite), argillaceous material or clay or oxides of iron 



