36 New York State Museum 



Eolian sands may be pure quartz, pure gypsum as some 

 of the desert sands of the southwest, sometimes pure lime, 

 such as eolian coral sands. Volcanic materials of arena- 

 ceous texture form volcanic sandstones. Sandstones and 

 breccias of volcanic origin are generally included under 

 igneous rocks. 



Sandstones have a general distribution all over the 

 world, and they are well known through their use as 

 building stones. The red and brown sandstones, because 

 of the insoluble nature of the iron oxide cement, make 

 the best building stones in moist climates. 



SJtales, lime mudrocks etc. are mudstoncs in which the 

 grains are less than 0.05 mm in diameter. Clay is a com- 

 mon constituent of such rocks but it may be entirely want- 

 ing. Quartz flours, lime flours and lime muds make up 

 some of these rocks. Shales are formed from compacted 

 muds and clays, have a more or less thinly laminated 

 structure and split into thin layers. The particles in 

 shales are too fine to be seen with the eye, sometimes with 

 a lens. While the bulk of the material composing shales 

 is kaolin and related substances, sometimes with white 

 mica, tiny fragments of quartz and other minerals may 

 also be present. With the increase of the quartz content 

 the shales grade into sandstones. Metamorphosed shales 

 become slates or phyllites and develop a slaty cleavage 

 (page 56). Some shales are highly fossiliferous, and in 

 this case they are calcareous shales. Red shales derive 

 their coloring from the presence of oxide of iron. Where 

 there is much quartz flour silicious shales are formed and 

 these upon weathering break up into irregular pieces, 

 some pencillike in form. Pyrite shales contain iron pyrite 

 which upon weathering causes the splitting up of the 

 rock. Carbonaceous material, as coaly matter and oil, 



