Common Rocks and Their Determination 



By W. A. Tarr, 



University of Missouri. 



SECOND PAPER. 



A working knowledge of the more common minerals would 

 naturally lead us to study their associates, the rocks, which are 

 composed of minerals. Some are composed of a single mineral 

 and some of several. The opportunities for studying rocks are 

 even greater than for studying minerals, as the former are pres- 

 ent in every locality in abundance, although the actual number of 

 varieties may not be many. The hills and the valleys, the moun- 

 tains and canyons are features on the earth's surface which are 

 formed by the two agents, wind and water, which cut away parts 

 of the rock. 



A stroll through the fields or a walk along a nearby creek will 

 usually reveal several kinds of rocks. They will be found out- 

 cropping along the banks and small gullies and in the creek beds. 

 Sometimes a very hard layer will cause a small fall in the brook. 

 Along railroads there are usually good exposures of rocks. In 

 some flat regions where the soil is thick there may not be very 

 good exposures, but wells in the neighborhood or a large river 

 may supply material for the students to work upon. Exchanging 

 specimens with the children of another region will give a greater 

 variety and number of rocks to work with and study, though 

 in this case they will not have the benefit of seeing the rocks 

 in place. 



But suppose we do see the rocks and minerals in the fields 

 what do they mean to us unless we know something about them ? 

 Nothing at all. But if we know that one, for example, is an 

 igneous rock, and that it probably came from many miles below 

 the earth's surface, we may infer something about the rocks on 

 the interior of the earth. Or limestone with its many fossil 

 shells tells us of a time, possibly millions of years ago, when ani- 

 mals lived in those same shells which now go to make up the 

 limestone. And we find fossils of animals and plants in other 

 rocks, in shales and sandstones and in coal, and these all tell of 

 times long since past, when these forms of life flourished on the 

 earth. And these sedimentary rocks tell us also that the sea or 

 some large lake has been over the very place where we are stand- 



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