Ch. 1] CLASSIFICATION 31 



Chapter 29.) The mineral constituents of sediments dissolve or grow 

 to such an extent that as time goes on the grains are locked more 

 firmly together, with resulting increase in strength of the rock. A 

 great variety of minerals are known to be generated in sediments, but 

 the controlling factors of formation of new minerals are not yet com- 

 pletely known. (See Chapter 25 in this symposium, and Pettijohn, 

 1949, pp. 476, 500.) 



CLASSIFICATION 



Nomenclature of sediments is a complicated subject, principally 

 because so many different attributes are used in classifying sediments. 

 Many systems of terminology have been described among which may 

 be mentioned those of Allen (1936), Casagrande (1947), Grabau 

 (1913), Jenny (1941), Krumbein and Pettijohn (1938), Krynine 

 (1940), Pettijohn (1949), Rutledge (1940), Twenhofel (1932, 1937), 

 Udden (1914), Wentworth (1935), and Wentworth and Williams 

 (1932). Most systems represent a combination of descriptive and 

 genetic factors. However, in so far as possible, a classification should 

 be descriptive, because a genetic classification is based on some sort 

 of conception of the mode of origin of the deposit based on descriptive 

 properties. Three main groups of sediments are generally recognized: 

 (1) clastic deposits, composed principally of particles that have been 

 transported mechanically and then deposited; (2) chemical deposits, 

 composed mainly of materials precipitated from solution; and (3) 

 organic deposits resulting from the activity of organisms. 



Some deposits such as limestones overlap all three groups. Lime- 

 stones can be composed of particles transported by water, ice, or wind 

 from some limestone area and then deposited ; they may form directly 

 as a result of chemical action; or they may result from the metabolic 

 process of organisms. The end products in all three processes may be 

 similar. The mode of origin, therefore, might be ascertained only 

 after careful study, and perhaps not then. Siliceous deposits may also 

 be difficult to classify, particularly after they have been subjected to 

 percolating waters which have dissolved and reprecipitated various 

 materials. However, most deposits of chemical origin, such as salt, 

 potash, gypsum, chert, and some iron deposits, as well as organic de- 

 posits of coal, oil, shale, and even petroleum, can be classified so clearly 

 that no further discussion is needed here. The principal confusion ex- 

 ists among the clastic or mechanical deposits. 



The clastic deposits are classified mainly upon the basis of the 

 grain size of the major constituents. The names gravel, sand, silt, 



