TAKK 



COMMOX ROCKS 



35 



Figure 2. — ^Sedimentary Rocks Showing Horizontal Straiiiication, 

 Penn. (Campbell and Battis, U. S. Geol. Sur.) 



called a sandy shale and might even pass into a sandstone, and 

 this might in turn, by an increase in the size of the particles 

 composing it, grade into a conglomerate. Likewise, we have 

 sandy limestones and calcareous sandstones. A very fair esti- 

 mate of the amount of the various materials in the rock may 

 be obtained by carefully noting the hardness, for the presence 

 of the sand grains would increase the hardness of a shale or 

 limestone, or the calcareous cement in a shale would make it 

 liarder. Treatment with acid also suggests the amount of the 

 impurity. But taken as a whole, the majority of the sediments 

 are nearly pure so that not a great deal of difficulty will be ex- 

 perienced in determining them. 



The sediments are of a great many colors, white, gray, red, 

 yellow, brown, green, black, and many others are very common. 

 The red, yellow, green, and brown are due to iron oxides. The 

 red is due to hematite and the yellow to limonite (iron rust). 



