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Vol. 9 February, 1913. No. 2 



Common Rocks and Their Determination 



By W. a. Tarr, 

 University of Missouri. 



THIRD PAPER. 



The sedimentary rocks constitute the second class. The word 

 sedimentary is apphed to these rocks because they are lain 

 down or deposited in water. This entire class of rocks is de- 

 rived from other pre-existing- rocks. All rocks when exposed 

 to the various agents of weathering, such as the air, water, 

 changes of temperature, acids, etc., at the surface, alter to sim- 

 pler minerals that do not decompose under the conditions that 

 exist there. The most of the minerals so formed are kaolin 

 (the mineral which clay is very largely composed of), calcite, 

 limonite, salt (which goes into solution and is carried down to 

 the sea or to lakes) and some other less important minerals. 

 Quartz, as mentioned above, does not alter except very slowly. 

 We can only trace brietly, in this short paper, the history of these 

 various products before their deposition, however interesting it 

 may be. The wind and the water work upon the rock debris and 

 the various particles are separated, usually according to size. 

 The particles that are small enough are carried away by the 

 streams to the sea, along with the material that was taken into 

 solution. This material carried by the streams is commonly 

 called sediment and on reaching the quiet waters of the sea or 

 lake it settles down to the bottom. The first material dropped 

 is the sand, which will become standstone later on : the fine ma- 

 terial will form the mud, which will become shale; while the 



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