SUPERIOR SANDSTONES. 417 



For this reason that enumeration may admit of 

 being brief. 



In the forms, dimensions, and divisions of 

 the strata, the upper sandstones resemble the red 

 sandstone already described. The masses, or 

 collections of beds, often also attain to very con- 

 siderable dimensions, without the intervention of 

 any alternating* substances. In the island of 

 Rasay, for example, they reach to a thousand 

 feet and upwards in thickness. 



Like the red sandstone also, these strata alter- 

 nate with smaller intervening beds of shale, and 

 of limestone ; and, in a few instances, with beds 



of clay, and of sand. In some cases, these alter- 







nating substances are considered as subordinate 

 to some mass or series of sandstone strata ; and 

 thus there is described a formation or particular 

 association, which seems to be sometimes of a 

 very arbitrary, if not of an imaginary nature ; 

 since, in other instances, the same alternations of 

 these calcareous or schistose rocks, under dif- 

 ferences of dimension, are held to constitute dis- 

 tinct for mationS) or to separate one association of 

 sandstone strata bearing some local title, from 



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