180 FIRST YEAR COURSE IN GENERAL SCIENCE 



The principal sedimentary rocks are conglomerates, sand- 

 stones, shales, and limestones. 



Conglomerates, or " pudding-stones," are rocks made from 

 beds of sand mixed with pebbles and stones. 



FIG. 90. A SANDSTONE QUARRY 



Find in this picture evidence: 1. That layers of sediment are not all 

 of the same thickness. 2. That these strata have not been much dis- 

 turbed by uplifts. 



Sandstones are made from sand beds, the grains of which 

 are mainly quartz. They are the most common rocks. A 

 well-cemented conglomerate or sandstone makes a very 

 durable rock. The so-called brownstone, widely used both 

 as building and as trimming stone, is a sandstone in which the 

 cement is an oxide of iron that gives the stone a reddish- 



