HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 535 



ter of life on land and sea, and the changes in the surface of the laud 

 itself from the earliest time down to the most recent. (See Geikie, pp. 

 158-176.) 



As will be readily comprehended when we cousider from what a multi- 

 tude of materials the fragmentai rocks have been derived, the amount 

 of assorting, admixture with other substances, solution, and transpor- 

 tation by streams these materials have undergone, they can not be 

 classified by any hard and fast lines, but one variety may grade into 

 another, both in texture aud structure as well as chemical composition, 

 almost indefinitely. Indeed many of them can scarcely be considered 

 as more than indurated mud, aud 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 

 fragmentai d6bris from plant aud animal life, they are here divided into 

 two maiu groups, each of which is subdivided as below : 



(A) Eocks formed by mechanical agencies and mainly of inorganic 

 materials. 



1. The arenaceous group — Psammites. — Sand, gravel, sandstone, con- 

 glomerate, and breccia. 



2. The argillaceous group — Pelites. — Kaolin, clay, wacke, shale, clayey 

 marl, argillite. 



3. The calcareous group. — Arenaceous and brecoiated limestones. The 

 rocks of this group are often in part organic and in part chemical de- 

 posits. Only those are placed here in which the fragmentai nature is 

 the most pronounced characteristic. 



4. The volcanic group. — Fragmentai rocks composed mainly of ejected 

 volcanic material. Tuffs, lapilli, sand and ashes, pumice dust, trass, 

 peperino, pozzulano, etc. 



5. The ferruginous group. — Fragmentai iron ores, hematites, limon- 

 ites, etc. 



(B) Rocks formed largely or only iu part by mechanical agencies 

 and composed mainly of the debris from plant and animal life. Orgau- 

 agenous. 



1. The siliceous group. — Infusorial earth. 



2. The calcareous group. — Fossiliferous and oolitic limestone, marl, 

 shell sand, shell rock. 



3. The carbonaceous group. — Peat, lignite, coals, bitumen, oil shale, etc. 



4. The phosphatic group. — Phosphatic sandstone, guano, coprolite 

 nodules. 



A. — Rocks composed mainly of inorganic materials. 



1. The arenaceous group — Psammites. — Arenaceous from the Latin 

 arenaceus, sandy or sand-like. Psammite from the Greek Wa/u/AiTtjs 

 sandy. 



These rocks are composed mainly of the siliceous materials derived 

 from the disintegration of older crystalline rocks and which have been 



