STRATIFICATION 119 
been formed by sea-weeds waved to and fro by eddying waters, 
the fronds of the algz brushing the surface of the sand or 
mud, and thus drawing or etching curved patterns. Others, 
again, may represent the trails made by floating alge or by 
the tentacles of a jelly-fish. Various surface-markings which 
mimic organic structures and have been given generic and 
even specific names are probably often of mechanical origin, 
formed either during or after consolidation of the rocks in 
which they occur. 

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