36 GEOLOGY 



decay. As a matter of fact, some decay usually accompanies 

 mechanical breaking. The sediment which contains some of the 

 feldspar of granitic rock is called arkose. Arkose represents in- 

 complete decomposition of the parent rock. 



Cementation of sediment into solid rock. After sediments such 

 as gravel, sand, mud, etc., are deposited in the sea or elsewhere, 

 they may be cemented into solid rock by the deposition of mineral 

 matter held in solution in water. This cement binds the pebbles, 

 the grains, and the smaller particles together, much as lime binds 

 sand together in mortar. The cemented gravel makes conglom- 

 erate, or if the pieces of rock are angular, breccia; the cemented 

 sand makes sandstone; and the cemented mud makes shale. These 

 are common sorts of sedimentary rock. The cementation may 

 take place while the 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 up shells, coral, etc., might also be regarded as clastic. 

 In contrast with igneous rocks, clastic rocks are made up of par- 

 ticles of other rock, particles which were once separate and distinct, 

 but now bound together by some sort of cement. The original 

 constituents touch one another, but do not interlock, as do the 

 crystals of igneous rock. 



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

 animals are frequently imbedded in them. If the shells or their 

 forms are preserved, they make a record of the kinds of life that 

 lived when the sediment was being deposited. If the sediments 

 were deposited in lakes or on land, it is the shells or other relics of 

 freshwater or land life which are found in them. All such relics 

 of past life are fossils. 



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

 It has already been noted, in connection with the decay of rocks, 

 that some of the compounds formed when rock decays are soluble. 

 A considerable part of the materials dissolved are carried in solution 

 to the sea. Here some of them are extracted by the animals and 

 made into shells or other hard parts. When the animals die, their 



