Handbook of Paleontology 23 



formed there is a certain amount of cementation by 

 mineral matter. The weight of younger sediments 

 upon the older ones forces the grains in the older sedi- 

 ments closer together causing them to adhere more 

 or less firmly. Sands thus indurated form a soft sand- 

 stone known as freestone. Finer sediments, such as 

 clays, become more strongly consolidated by this pro- 

 cess and form shales. 



Consolidation by cementation is a very common 

 process, particularly at greater depths. Percolating 

 waters dissolve certain mineral constituents of the 

 sediments and redeposit them farther down to act as 

 a binder between the grains. Temperature increases 

 with depth and increases the chemical activity of 

 water. Pressure is also an aid in consolidation by 

 cementation. The common cements binding together 

 the particles of sediments are calcite or carbonate of 

 lime, silica and iron oxides such as ferrous oxide, 

 hematite and limonite. In calcareous deposits carbon- 

 ate of lime is the chief cementing material, but it also 

 may bind together quartz grains in calcareous sand- 

 stones and pebbles in conglomerates. Where silica is 

 the cement sedimentary rocks are very hard, especially 

 if the material consists of quartz grains. Here the 

 silica is deposited around the grains so that the new 

 quartz and the quartz grains are continuous. As the 

 process is continued the grains finally become indis- 

 tinguishable and a bed of quartzite is formed. Ferrous 

 oxide has no distinguishing color, but hematite is red 

 and limonite is brown or yellow and their presence as 

 a cement gives the familiar yellow and brown colors 

 to our sandstones. The brownstone so familiar in 

 buildings is a sandstone so cemented. 



