AEENACEOUS EOCKS: PSAMMITES 113 



have -undergone, they cannot be classified by any hard and fast 

 lines, but one variety may grade into another, both in texture 

 and strnctnre as well as in chemical composition, almost indefi- 

 nitely. Indeed, many of them can scarcely be considered as 

 more than indurated muds, and only very general names can 

 be given them. 



Accordingly as these rocks consist of mechanically formed 

 inorganic particles of varying composition and texture, or of 

 the more or less fragmental debris from plant and animal life, 

 they are here divided into two main groups, each of which is 

 subdivided as below: 



I. Rocks formed by mechanical agencies, and mainly of in- 

 organic materials. 



(1) The Arenaceous group — Psammites: Sand, gravel, sand- 

 stone, conglomerate, and breccia. 



(2) The Argillaceous group — Pelites: Kaolin, clay, wacke, 

 shale, clayey marl, argillite. 



(3) The Calcareous group — Arenaceous and breceiated lime- 

 stones. The rocks of this group are often in part organic, and in 

 part chemical deposits. Only those are considered here in which 

 the fragmental nature is the most pronounced characteristic. 



(4) The Volcanic group — Fragmental rocks composed mainly 

 of ejected volcanic material: Tuffs, lapilli, sand and ashes, 

 pumice-dust, trass, peperino, pozzuolano, etc. 



II. Rocks formed largely or only in part by mechanical agen- 

 cies and composed mainly of the debris from plant and animal 

 life. 



(1) The Siliceous group — Diatomaceous earth. 



(2) The Calcareous group — Fossiliferous and oolitic lime- 

 stone, marl, shell-sand, shell-rock. 



(3) The Carbonaceous group — Peat, lignite, and the coals. 



(4) The Phosphatic group — Phosphatic sandstone, guano, 

 coprolite nodules. 



(1) EOCKS COMPOSED MAINLY OF INOEGANIC MATEEIAL. 



(1) The Arenaceous Group: Psammites. — Arenaceous, from 

 the Latin arenaceous, sandy or sand-like; psammite from the 

 Greek (papLjutry]?, sandy. 



These rocks are composed mainly of the siliceous materials 

 derived from the disintegration of older crystalline rocks which 



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