SANDSTONES 381 



SANDSTONES 



Sandstones consist of grains of sand which are bound together 

 by a cementing material. 



The grains raa.y be of varying sizes, from almost impalpable 

 dust to small pebbles, and may be angular or more or less rounded 

 in form. The cementing matter also may vary greatly in its 

 nature. From this variation, both in the grains and in the cement, 

 there is an almost endless gradation in the kinds of sandstone. 



Quartz is the essential constituent, but with it there may be 

 feldspar , mica, calcite, pyrite, glauconite, clay or other minerals, 

 and rock fragments common to stone of sedimentary origin. 

 These accessory materials often give character to the mass, and 

 make a basis for a division into feldspathic, micaceous, calcareous 

 sandstones, etc., as one or another of them predominates. 



The texture of the mass also is subject to a wide range of varia- 

 tion, from fine-grained, almost aphanitic, to pebbly sandstone, or 

 conglomerate, or a brecciated stone in which the component parts 

 are more or less angular. 



Some of the brown sandstones of the Triassic age, quarried 

 near Haverstraw, are such conglomeratic and brecciated sand- 

 stones. Accordingly as the grains are small or large the stone is 

 said to be fine-grained or coarse-grained 



The variety of the cementing material also affords a basis for 

 classification. Silicious sandstones have the grains bound to- 

 gether by silica. They consist almost exclusively of quartz, and 

 grade into quartzite. The ferruginous varieties have for their 

 cement an oxide of iron, often coating the grains and making a con- 

 siderable percentage of the whole. The iron is usually present as 

 ferric oxide. Calcareous sandstones are marked by the presence 

 of carbonate of lime. "When it exceeds the quartz in amount, the 

 sandstone becomes a silicious limestone. In the argillaceous 

 varieties, the binding material is a clay, or an impure kaolin. 



The cementing material determines ia most cases the color. 

 The various shades of red and yellow depend upon the iron 

 oxides ; some of the rich purple tints are said to be due to oxide 

 of manganese. 



The gray and blue tints are produced by iron in the form of 

 ferrous silicate or carbonate. Bv an irregular association of 



