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PRINCIPLES OF STRATIGRAPHY 



forming in quieter water will shade off into calcareous sediments, 

 and may even be entirely replaced by limestones of this type. 

 Where coral reefs or shell heaps are forming off shore, the clastic 

 derivations of these will become commingled with, and shade off 

 into, the terrigenous sediments near shore. As explained in 

 Chapter X, the coarsest fragments will remain near the reef, the 

 calcarenytes coming next, and shading off into the finest calcilutytes. 

 These calcilutytes may be gradually replaced shoreward by siliceous 

 or argillaceous Tutytes, or they or the calcarenytes may grade di- 

 rectly into the silicarenytes. Where this latter occurs, we have a 

 seaward change from pure quartz sand (silicarenyte), through cal- 

 ciferous sandstone (calcareous silicarenyte) and siliceous calca- 

 renytes to pure calcarenytes. This change is probably more 

 often observed in the Palaeozoic series than the change from siliceous 

 to argillaceous sediments. (Fig. 145.) 





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Fig. 145. Diagram showing regular marine progressive overlap; a basal 

 sandstone is present, but this grades upward and seaward into 

 calcareous deposits. The differences between sections A and C 

 are readily seen. 



Older examples. The type of overlap here described seems to 

 have been by far the most general as recorded in both Palaeozoic 

 and Mesozoic deposits. An illustration is seen in the basal Ordo- 

 vicic sandstone of Ontario, which on Lake Huron lies at the base 

 of the Chazy series, but farther northeast is basal Trenton. The 

 basal Cambric sandstone of Sweden also varies in age from Lower 

 to Tapper Cambric, though there is probably a series of unre- 

 corded intervals during which retreat and erosion took place with- 

 out the deposition of a basal sandstone by the readvancing sea. 

 From the evidence of the dreikanter and other structural features 

 it is known that this basal bed represents an old residual sandy 

 covering of terrestrial origin, subsequently encroached upon by the 

 sea. It is not improbable that wherever basal sandstones occur, 

 extending upward through such a long series, as the entire Cam- 

 bric in which there are, moreover, stratigraphic breaks, these sand- 



