360 ROCKS. > 



to this quality and the cellules, which give it a very rough 

 surface. In the best stones for wheat or corn the cavities 

 about equal in space the solid part. The finest quality 

 comes from France, in the basin of Paris and some adjoin- 

 ing districts. 



The stones are cut into wedge-shaped parallelopipeds called 

 panes, which are bound together by iron hoops into large 

 millstones. The Paris buhrstone is from the tertiary forma- 

 tion, and is therefore of much more recent origin than the 

 quartz rock above described. 



Buhrstone of good quality is abundant in Ohio, and others 

 of the western states. It is associated there with proper 

 sandstones, as more particularly mentioned on page 346. 



The quartz rock of Washington, near Pittsfield, Mass., is 

 in some parts cellular, and makes good millstones. 



A buhrstone occurs in Georgia, about 40 miles from the 

 sea, near the Carolina line ; also in Arkansas, near the 

 Cove of Wachitta. 



SANDSTONES. GRIT ROCKS. CONGLOMERATES. 



Sandstones consist of small grains, aggregated into a com- 

 pact rock. They have a harsh feel, and every dull shade of 

 color from white through yellow, red and brown to black. 

 Many sandstones are very compact and hard, while others 

 break or rub to pieces in the fingers. They usually consist 

 of siliceous sand ; but grains of feldspar are often present. 

 In many compact sandstones there is much clay, and the 

 rock is then an argillaceous sandstone. 



Sandstones are of all geological ages, from the lower Silu- 

 rian to the most recent period. The older rocks are in 

 general the most firm and compact. The " old red" sand- 

 stone is a sandstone below the coal in age ; while the so 

 called " new red" is more recent than the coal. But these 

 terms are of indefinite application out of Great Britain, and 

 are not now used in this country. Red sandstone, when 

 used as a building material, is often called freestone. 



Grit rock. When the sandstone is very hard and harsh, 

 and contains occasional siliceous pebbles, it is called a grit 

 rock, or millstone grit. 



- Conglomerates. Conglomerates consist mostly of pebbles 

 compacted together. They are called pudding stone when 

 the pebbles are rounded, and breccia when they are angular. 

 They may consist of pebbles of any kinds, as of granite. 



