294 



Our Surroundings 



ing agents than upon the grains of which they are composed. 



The grains are generally bits of quartz in various shapes, round 



or angular. Sometimes they are formed of fine particles of other 



rocks. If the binding agent 

 is a material like clay, liable 

 to disintegrate under the action 

 of water from rainfall, the 

 stones are not durable, but if 

 held together by a material 



fv, 4iJH-iS;V; : I containing silica they are both 



.;/ '.">. strong and durable and are 

 among the most valuable of 

 natural building materials. The 

 Potsdam sandstone of St. 

 WATER RUNNING OVER SANDSTONE Lawrence county, New York, 



is a notable example of this class. 



Excellent sandstones for construction purposes come from 

 other parts of tl^e United States. Among them are the Ohio free- 

 stones and the brown and red freestones of the Atlantic states and 

 of the eastern slopes of the Rocky mountain system. Beds of sand- 

 stone of a red-brown and gray color, located in western New York, 

 at Medina, provide high grade building and paving stone. Blue- 

 gray sandstones are extensively quarried in Albany, Green, and 

 Ulster counties in New York 

 state, and to some extent in 

 Pennsylvania. These are 

 known as bluestone or flag- 

 stone and are well adapted for 

 pavements, steps and sills. 



Limestone. Limestones, 

 composed mainly of the min- 

 eral calcite, form beds of rock 

 of a gray, blue, red, or black 



color. These rocks usually ,, T ,, r T 



3 WATER WEARING AWAY LIMESTONE 



contain some impurities, as 



iron, clay, or silica. Chalk is a soft limestone, commonly 



white, made up of the limy skeletons of minute creatures. 



