36 NATURE STUDY REVIEW [9 : 2— Feb., 19i:5 



The various shades of these are due to mixtures of the oxides. 

 The gray and black colors are due to carbonaceous matter in 

 the rock. The color of the fresh rock is usually different from 

 that of the altered part at the surface. This change in the color 

 is very noticeable in the quarries where the fresh and altered 

 rocks are exposed. Various minerals are occasionally found in 

 the sediments. Some of the most common are the chert and 

 flint in limestone, pyrite in shales, limestones and coal, quartz 

 crystals in cavities in the limestones, shales and sandstones, 

 gypsum crystals in shales and coal, and limonite and hematite 

 in shales, sandstones and limestones. 



The character of the underlying rock determines the kind of 

 soil above it. The shales give rise to a clayey soil as would 

 naturally be expected, while the sandstones make sandy soils. 

 Limestone generally decomposes by the calcium carbonate going 

 into solution and any insoluble impurities in it remaining behind 

 as a residual soil. 



Limestones and dolomites are calcium carbonates, and cal- 

 cium-magnesium carbonates respectively. They are usually dense 

 fine-grained rocks (see Fig. 9) although dolomites are often 

 porous and limestones, rarely. They break with a sub-conchoidal 

 fracture (see Fig. 9). Both rocks may contain a great many fos- 

 sils, and when they do they are often called fossiliferous lime- 

 stone or dolomite. Sometimes the amount of organic matter in 

 the limestone is high and then it is called carbonaceous lime- 

 stone. Lithographic limestone is a very dense, fine-grained lime- 

 stone used in lithographing. There are numerous other varieties. 

 Limestones and dolomites occur in layers or beds often hundreds 

 of feet thick. There is a series of limestone beds over a mile 

 thick in the western part of the United States. Shales, sand- 

 stones and sometimes conglomerates are found with limestones. 

 Occasionally valuable ores are found in them, but they came 

 from associated igneous rocks. In the following arrangement of 

 the sediments it is seen that the carbonate rocks originated 

 through organic processes. This is probably true in many in- 

 stances but not always as some limestones and dolomites are 

 probably of chemical origin. The uses are the same as those 

 given for calcite in the paper on minerals. 



The shales and clays consist of various minerals but the chief 

 ones are kaolin and quartz. The particles are very fine and are 

 deposited in quiet waters, hence shales are dense fine-grained 

 rocks. Shales occur in beds, the layers of which vary in thick- 

 ness from a sheet of paper to many feet. Some shale forma- 



