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conceived to comprise the sandstones, as the 

 limits between these and the former are un- 

 definable. In this latter case however, the 

 appearances may equally be ranked under 

 varieties of texture. 



In the aggregate structure, the parts may 

 consist of one, or of more rocks, or of these 

 intermixed with quartz, or of quartz alone. 



These different substances are sometimes 

 united without any apparent general inter- 

 medium ; at others, a common base appears to 

 pervade and cement the whole. 



The sizes of the fragments vary, from the 

 finest sand to a diameter of many feet ; but they 

 are generally limited to very moderate di- 

 mensions. 



In the finer aggregate structures, an average 

 general size commonly pervades all the parts ; 

 in the larger, or coarser, fragments of all sizes 

 are united in the same mass. 



In some examples, the parts are rounded, as 

 if by previous attrition; often, also, bearing 

 other marks of the action of air or water. In 

 others, the fragments possess more or less of that 



