514 PRINCIPLES OF STRATIGRAPHY 



cotton soil (regur) of India and in the black earth (tchcrno::om) 

 of Russia. Such deposits may subsequently harden into black, car- 

 bonaceous shales. Examples of such shales are probably to be 

 found in the Chattanooga black shale of eastern Alabama, etc., of 

 Lower Mississippic age. This in places includes thin layers of 

 coal, and in other respects bears evidence of having been a former 

 residual soil, the black color of which is due to the abundance of 

 partially decayed land vegetation. 



Burial of Peat Deposits. 



Peats formed near the coast, even when of fresh-water or 

 swamp origin, may be buried by a subsidence of the land and a con- 

 sequent transgression of the sea. Such subsidence may be so slow 

 that salt-water or marsh peat may transgress over the fresh-water 

 peat, as in the case of the Massachusetts coast (see ante, p. 493). 

 Where subsidence is inore rapid, fresh-water peat may be covered 

 by marine sands, as in the Bay of Morlaix, Brittany (Finistere), 

 where, according to Cayeux, two layers of peat with Arundo 

 phraqmites are separated and again covered by marine sands. In 

 the fiat, maritime plain of Pas-de-Calais, a section now three meters 

 above the sea-level shows : 



Marine sand with Cardimn cdule, i meter. 



Marine clay with Hydrohia ulvcc. i meter. 



Peat. 



Marine sand with Cardium edule. 



In the mouth of the Shelde similar deposits of peat, i to 1.5 

 meters thick, are enclosed between sediments with marine organ- 

 isms. At Cotentin, in Normandy, 20 meters of peat are overlain by 

 marine sands, and similar deposits are found in other parts of 

 France, in Belgium. Holland and elsewhere. As shown, however, 

 by the deposits of Anticosti, the presence of such marine organisms 

 in the strata covering the peat need not always indicate subsidence 

 after the formation of the peat. 



The section made by the river Tay in the Carse lands of south- 

 eastern Scotland shows a peat bog now forming the river bed and 

 covered by about 17 feet of alluvial material, which near the top 

 contains cockles, mussels and other marine forms. The peat of 

 this region rests in part on alluvial sands and in part on marine 

 clays, and is itself of terrestrial origin. It is "highly compressed 

 and splits readily into laminae, on whose surfaces are small seeds 

 and wing cases of insects. As a rule, but not always, it is marked 



