Handbook of Paleontology 29 



character of the rock from which they were derived 

 may be determined and also something of the condi- 

 tions under which they were deposited. Residual 

 sands usually are composed of the minerals making up 

 the parent rock, but in material which has been trans- 

 ported for a long time only the more resistant minerals 

 are left. This is why quartz is so common a consti- 

 tuent of sands. When a deposit shows an abundance 

 of decomposable minerals a very dry climate is indi- 

 cated. In a warm, moist climate decomposable min- 

 erals in time decay and are dissolved so that a high 

 percentage of quartz is left. An abundance of mica 

 in muds and mud rocks indicates a continental deposit. 

 Other hard minerals besides quartz that make up sands 

 are magnetite and garnet, the source of which may 

 be such metamorphic rocks as gneisses and schists. 

 Granites are often the source of quartz; other igneous 

 rocks and schists of mica. 



The fragments making up clastic deposits may vary 

 in size from large boulders through pebbles, gravels 

 and sands to the finest muds and clays. The particles 

 and fragments may be angular, subangular or rounded. 

 Pebbles may be subangular or rounded and boulders 

 likewise. Some formations show scratched, faceted 

 pebbles ; others polished, faceted pebbles. Angular 

 particles are seen in residual sands, that is, material 

 where there has been little or no transportation to 

 round off the angles and sharp edges. Angular frag- 

 ments are found in talus and landslide debris. Vol- 

 canic sands and coarser fragments are angular, also 

 some ice-worn material. Where there is movement 

 within the earth solid rock may be crushed producing 

 the angular and subangular fragments which form the 



