Handbook of Paleontology 27 



to a depth of 100 fathoms. The bulk of the material, 

 chiefly sands and muds, deposited over this area are of 

 land origin. Materials are brought to the sea by rivers, 

 glaciers, icebergs etc. ; by the wind, often from great dis- 

 tances ; by the erosion of the sea shore by the waves ; and 

 by the scouring action of ocean currents. Going out to 

 sea the material becomes finer and finer due to the sort- 

 ing action of the water until finally only the finest muds 

 are found. The zone of the finer muds is between the 

 100 and 1000 fathom depths, but the lower limit of the 

 muds often extends beyond the 6000-foot line. Here are 

 found blue, green and red muds and gray muds composed 

 of volcanic ashes. Beyond this depth is the profound 

 abyss where are found the various calcareous and sili- 

 cious oozes and the red clay, chiefly of volcanic origin 

 (fine dust), which is characteristic of the greatest depths. 

 Some marine deposits, such as the limestones, are of or- 

 ganic origin. These deposits have been touched upon 

 above. The breaking up and redeposition of such deposits 

 makes them secondarily of mechanical origin. Marine 

 fossils present in rocks are a certain indication of the 

 marine nature of the deposits. 



Description of clastic rocks. Some of the more im- 

 portant rocks of chemical and organic origin have been 

 described above. Rocks composed of mechanical sedi- 

 ments, the clastic rocks, form the bulk of the sedimen- 

 tary rocks and are of the greatest importance. As al- 

 ready pointed out clastic rocks are composed of materials 

 which are fragments of preexisting rocks, igneous, meta- 

 morphic or sedimentary. The agent that transports these 

 materials may have the power to separate the heavier and 

 lighter fragments. This is true of material carried by the 

 wind or running water, and this process of separation of 



