SEDIMENTARY ROCKS 267 



Cementation of sediment into solid rock. After gravel, sand, 

 mud, etc., are deposited in the sea or elsewhere, they may be cement- 

 ed into solid rock by the deposition of mineral matter held in solu- 

 tion in water. This cement binds the pebbles, the grains, and the 

 smaller particles together, much as lime binds sand in mortar. The 

 ivnu-nted gravel makes conglomerate, or if the pieces of rock are 

 angular, breccia; the cemented sand makes sandstone; and the ce- 

 mented mud makes shale. These are common sorts of sedimentary 

 rock. The cementation may take place while sedimentation is in 

 progress, or at a later time. Conglomerate, sandstone, and shale, 

 made up chiefly of particles derived directly from other rock, are 

 clastic, rocks. Limestone may be broken up, and its particles 

 redeposited and cemented again into solid rock. Such limestone 

 is clastic, and limestone made of broken shells, coral, etc., is in some 

 sense clastic. In contrast with igneous rocks, clastic rocks are 

 made up of particles of other rock, particles which were once separate 

 and distinct, bound together by some sort of cement. The particles 

 touch one another, but do not interlock like crystals of igneous rock. 



When sand, mud, etc., are deposited in the sea, shells of sea 

 animals may be imbedded in them. If the shells or their forms are 

 preserved, they record the kinds of life that lived when the sediment 

 was being laid down. If the sediments are deposited in lakes or on 

 land, the shells or other relics of freshwater or land life may be 

 buried in them. All distinct relics of past life are fossils. 



Non-clastic sediments. Not all sedimentary rocks are clastic. 

 It has already been noted that some of the compounds formed when 

 rock decays are soluble. A part of the materials dissolved are 

 carried in solution to the sea, where some of them are extracted by 

 animals and made into shells or other hard parts. When the 

 animals die, their shells and other secretions are left behind. If 

 these are of calcium carbonate, they make limestone when cemented 

 together. Much, if not most, limestone is composed of the secre- 

 tions of organisms. 



The shells, coral, etc., may or may not have been broken up 

 before cementation. Limestone has many varieties, one of which is 

 chalk. Magnesium may replace the calcium in various propor- 

 tions, and if there is any considerable amount of magnesium, the 

 rock is dolomite. The dolomization of some limestone (fhe con- 

 version of CaCO 3 into CaMg(CO 3 )2) appears to have taken place 

 long after the limestone was formed, while in other (perhaps in 



