74 GEOLOGY. 



sometimes crumbles is employed in making glass, and in 

 sawing and polishing marble. The sand of our sand- 

 paper comes from this source. The buhrstone, which is 

 made into mill-stones, is quartz rock that is full of little 

 spaces or cells, these making the surface rough with sharp 

 minute edges crossing each other in every direction. 



151. Sandstones. These usually consist mostly of sili- 

 cious sand, some of the varieties of quartz rock approach- 

 ing very nearly to them in character. Sometimes there 

 is much clay in their composition, and then the rock is 

 called an argillaceous sandstone. Sometimes there are 

 silicious pebbles imbedded in the rock, making it in part 

 a pudding-stone, and in that case, if the rock be very 

 hard, it is called a grit rock or mill-stone grit. Some 

 sandstones that readily split into comparatively thin lay- 

 ers are much used for flagging-stones. Sandstones have 

 dull colors of various kinds, from white, through grades 

 of yellow, red, and brown, even to black, these being 

 caused, of course, by various substances combined with 

 the silex or quartz. When sandstone has a fine, even 

 grain, it is a very beautiful building material. More cau- 

 tion is necessary in selecting sandstone for building than 

 granite, because varying circumstances of exposure to 

 air, moisture, and heat have so much influence upon it. 

 The sandstone obtained chiefly from New Jersey and va- 

 rious localities in the valley of the Connecticut, now so 

 much used in almost all parts of the country to which it 

 can be carried by water, is generally an excellent mate- 

 rial. That used in building the Capitol at Washington, 

 which came from the Potomac, is, unfortunately, an infe- 

 rior article. 



152. Trappeaa Rocks. This appellation is given to 

 certain rocks which have such resemblance to each other 

 as makes it proper to group them together. The term 

 trap comes from a Swedish word meaning stair, the rocks 

 of this class often presenting to the eye an appearance 

 like steps. The term greenstone is applied to the com- 



