AEENACEOUS EOCKS: PSAMMITES 



115 



allowing it to absorb water, which, on freezing, brings about 

 the disintegration. The argillaceous cementing material, while 

 in itself inert, also permits a high degree of absorption, with 

 like results. Those sandstones cemented by silica, which there- 

 fore partake of the nature of quartzite (see p. 137), are by far 

 the more refractory. 



The following analyses will serve to indicate the consid- 

 erable range in composition of rocks of this class: 



Chemical Composition of Sandstones 



Constituents 



Silica (SiOa) 



Alumina (AI2OS) 



Iron oxides (Fe^Os) and (FqO) 



Manganese (MnO) 



Lime (CaO) 



Magnesia (MgO) ..... 



Potash (K2O) 



Soda (NaaO) . 



Loss 



Totals 



69.94% 

 13.15 



2.48 



0.70 



S.09 

 Trace 



8.30 



5.43 



1.01 



99.10% 



II 



84.40% 

 7.49 

 3.87 



• • « « 



0.74 

 2.11 



0.66 



99.41% 



III 



96.24% 

 0.56 

 1.28 



1.40 

 1.23 



0.56 



100.27% 



IV 



90 86% 

 4.76 

 L58 



« f • • 



0.15 

 0.59 

 1.06 

 0.45 



99.45% 



I. Brown Triassie sandstone: Portland, Connecticut. II. Gray sub-Carbo- 

 niferous sandstone: Berea, Ohio. III. Eed Carboniferous sandstone: Anan, 

 Scotland. IV. Cambrian sandstone: Siskowit Bay, Wisconsin. 



The table given on p. 145 will serve to show the close chem- 

 ical relationship existing between many rocks of this group, 

 and their metamorphic equivalents. 



The colors of sandstone are dependent npon a variety of 

 circumstances. The red, brown, and yellowish colors are dne 

 to iron oxides in the cementing constituent. Some of the dark 

 colors are due to carbonaceous matter. 



Many varieties of sandstone are popularly recognized. Cal- 

 careous^ ferncginous, siliceous^ or argillaceous sandstones are 

 those in which the cementing materials are of a calcareous, fer- 

 ruginous, siliceous, or argillaceous nature. The name arJwse is 

 given to a coarse feldspathic sandstone derived from granitic 

 rocks, with a minimum amount of loss of original material. Con- 

 glomerate or puddingstone is merely a coarse sandstone ; it differs 

 from ordinary sandstone only as gravel differs from sand. Brec- 



